All of which I find a bit bemusing. When I started sewing I didn't really understand that knits were a different beast to woven fabrics, and while I admit to many a mistake along the trial and error road, knits have never really scared me any more than any other sewing hurdle. I mean have you tried silk chiffon? satin? There's all manner of things out there that present unique and considerable challenges. Knits are not the monster many people seem to think, and I would really encourage you to have a go. Now that so many woven fabrics have added stretch, people are more familiar with sewing knits than they might think.
But since I get asked a lot about this topic I'll let you know what I know, for what it's worth.
Why sew knits?
While T-shirts and leggings can be cheap, they aren't always, and in terms of quality, home sewn is definitely better value. But there are an astonishing array of knit fabrics now on the market and you can use them to create some really interesting garments. Also, knit fabrics are very forgiving compared to wovens. By which I mean if you are a beginner or slacker sewer, you can fudge and cut corners easier. Knits often don't fray and so require less finessing and finishing and because knits stretch when you are wearing them there's a lot less pressure to get a precise fit for your body. Once you find a pattern or patterns you like, you can run up a bunch of T-shirts or leggings really quickly.
The tool kit
The tools required for sewing knits is really pretty light on. I started sewing knits on an ancient old Singer machine with nothing extra.
In the main you do require a machine with stretch stitches (stitches that can expand and contract with the stretch of the fabric) but basic stretch stitches are standard on pretty much all machines now. I have ten year old bottom of the range Janome and it has several more stretch stitches than I need.
At a base I tend to use the stitch that looks a bit like a back to front blanket stitch (15 above) - it seams but also throws threads over the cut edge of the seam allowance, the straight stitch (18) and very occasionally the one that looks a bit like a honey comb (16) that gives a slightly decorative edging for very narrow hems. There are also stitches especially for attaching narrow elastic as you would for bathers edges and lingerie - your sewing machine manual will tell you which ones they are.
It is most definitely worthwhile investing a few dollars in buying ball point or jersey needles for your machine because regular needles can skip the occasional stitch with knits. Also if every you find yourself unpicking stretch stitch (not recommended AT ALL) a regular needle will leave holes in a knit fabric but a ball point won't.
If you are going the deluxe tool kit you'd have an overlocker and use it for your seams instead of a sewing machine. A 4 thread overlocker does a very tidy job of seaming and finishing off the seam allowances that make garments look just like they came from the sweatshop, plus they are faster. But I sewed knits for 20 years without one, and they come with their own complications. In the picture above, the bright blue on the left is sewn using the stitch 15 on the sewing machine, the one on the right is sewn with an overlocker. It should be noted the one sewn on the machine is over 10 years old! On the right side of the fabric they look the same.
The super luxe professional add on is a coverstitch machine, which exists to give knits those really nice double row of stitching hems - see the darker blue T-shirt in the two shots above. I'd love one but couldn't justify the expense and a straight or decorative stretch stitch on a regular machine is a passable alternative.
Another option for a regular machine is to use a double needle (see brown stitches in above shot). This is a needle which has a single shank but twin needle tips coming off it. You thread 2 separate spools of thread for the top feed, one for each needle, and a single bobbin below. You then use a regular (non stretch) straight stitch and the bobbin thread catches both on the underside.
This gives you the double row of stitching on the right side, less fabric distortion, great stretchiness and a zig zag style finish on the raw edge underneath. The main downside, aside from needing to sew hems right side up, is that once the fabric has been stretched, the fabric between the two rows of stitching tends to bubble up (called tunnelling) and can't be brought back to flat. Despite this draw back, I think this is a neat finish.
You can also bind or cuff edges to conceal stitching.
Of course raw edges works just fine with most knits as they don't fray.
And that's it.
Different types of knit and stretch
Knits are constructed in a range of different ways with different fibres. The more stretch and slip, the more attention you have to pay. Standard cotton jersey - T-shirt material - without any lycra or spandex in a medium weight behaves fairly similarly to a woven a fabric. It doesn't stretch that much and it only stretches across the length of fabric (selvage to selvage). On the other end of the scale is shiny lightweight nylon lycra - stretches in both directions, slips around and stretches a lot so sewing seams can require a lot of attention to make sure they stay aligned while you sew. Chose an easier fabric for your first attempts and don't stress too much because a fair amount of seam misalignment will never show on the finished garment.
That sounds way more high falutin' that it should. Since I just taught myself through trial and error I am sure there is much more to doing this super well, but what I do works good enough.
I use much smaller seam allowances when sewing stretch than wovens - say 5mm instead of 10 or 15mm. Since fraying isn't generally an issue, extra seam allowance isn't required and the stabilisation it provides for wovens acts to dampen the fluidity of the knit.
I sew straight seams where the stretch of the 2 pieces of fabric is matched (say the side seam on a T-shirt) just the same as I would any woven fabric (except using a stretch stitch). Where I am sewing curves, or unmatched stretch, like a sleeve head, I use a lot of pins and go slowly and use my fingers right in near the foot to hold and feed the fabric to keep the alignment good. If I start to lose alignment, I pull or stretch the fabric to bring it back into alignment before the next pin, even if that involves a bit of distorting. Small distortions really don't show, but big ones do!
There is no need to stretch the fabric as part of sewing or hemming. The stretch stitch provides the give and stretching while sewing will prevent it from retracting its stretch and sitting flat. There are a few tricks I have heard people use, like using tissue paper over and under the fabric to decrease the drag while you are sewing, but in general I find these fiddly and not much help.
I started making swimwear with cotton lycra because when it first came on the market it wasn't common and it was particularly hard to find nice stuff in larger sizes. I'll confess and say I would start with a basic outline from an existing swim suit, cut it off a bit bigger and then gradually make it smaller until it fitter right! Since bathers use quite a small amount of fabric, catastrophic failure isn't such a great tragedy.
I now know a bit more now, like using lining with lightweight lycra to add stability, using swimwear specific elastic and how to sew it into a hem. I also know that with sun protection considerations, swimwear choices are much broader than simply a one piece or a bikini. But I like to retain my basic premise that it is just fabric designed to cover you - how hard can it be?
My recent two piece consists of a rash vest I made using a T-shirt pattern I like and some lycra shorts made using a pants pattern I have used lots before that relies on stretch in the fabric rather than a closure ('Jazz Pants' from Ottobre). I had to go back and add some elastic in the waist since the lycra was not so firm once wet! The top is a very lightweight, matt finish almost papery feeling lycra, and the bottom is a heavier weight classic nylon lycra. If I was making a swim suit I would definitely prefer the latter for the strength and firmer fit (more flattering too!), but the lighter weight is great in a rash vest where you need to move around.
This suit didn't require elastic, but here's 2 examples of edges which have elastic. The top one has the elastic sewn to the edge of the fabric and then turned and a hew sewn. The bottom darker one has a binding on the edge with the elastic threaded through. This is much neater but more work!
Good quality lycra, preferably intended for swimwear, is important to resist fading and the corrosive effects of chlorine and salt water. Every single one of my home made swimsuits have been retired due to fabric failure (mostly fading or using too lightweight fabric for a one piece) rather that shoddy construction or style issues.
I recently came across this site (www.patternschool.com), and if you are looking to start making swimwear at all seriously I would thoroughly recommend you trawl it in detail. It is an amazing repository of info about stretch fabric, swimwear, making patterns, sewing stretch, you name it!
So I don't know whether any of that was helpful? Let me know if there are specific things I could add in or questions you have.
Edited to add -
I neglected to really talk about sewing on banded edges, something one commenter thought was particularly scary. They aren't. Bands can be used to finish off edges such as the neck of a T-shirt or the cuffs of a sleeve or the waist of yoga pants. The purpose of bands is to pull the fabric in - either to stabilise an edge that may stretch out a lot (say the neck of a T-shirt) or to create a blousing out effect (the bottom hem of a windcheater/sweatshirt) or to create a tighter fitted section of a garment (the waist of yoga pants).
In the olden days, all bands were done in ribbing - a much stretchier version of the base garment fabric - but there is no reason why you can't use the same base fabric for bands too. The basic principle is the same: the band is smaller than the part of the garment it is attaching to and you simply stretch the band out to fit and sew it on. The smaller the band and the more stretch the more of a retraction you will get in the finished edge, the more 'pucker'. You can of course alos use a band of the same size as the garment edge (see the photo above of the neck of the navy blue T-shirt).
Sewing on a band is not at all tricky unless you are going for a LOT of stretch, but visualising exactly how much stretch you want and getting the ratio right is hard. Even if you are using a pattern, the amount of stretch in your particular fabric will almost certainly be different than what the pattern used, so it might not come out as you expected. If you are very particular, you would do a test sample with scraps and adjust the size of the band. I know there are formulas for testing the stretch and recovery of fabrics, and rules of thumb for how much stretch you want, but I find trial and error works just fine!
The method for attaching a band is usually to sew the band ends together and fold it over right sides out. Mark quarter or eight parts around the band and match these up to the markings around the end of the garment where the band will go. Occasionally you may want to attach the band with the stretch applied unevenly (say on a neck band with a very angular curve) but mostly even is best (and easiest!).
Pin the band to the right side of the garment (band upwards when you sew) at each of the quarter or eighth markings. When the band is relaxed the garment will gave some slack between each pinned section. Working section be section, pull the band taut over the garment and put one more pin in. Now sew the two together. You will need to pull the garment between your two hands to stretch the band as you go, making sure you are in alignment at each pin.
When you are finished you may want to top stitch the edge of the garment close to the band to hold the seam allowance flat.