I’ve done my research and actually tried some of them but the end results are the same: wonky.
I’m using needles specifically for knit fabrics, I have used a knit foot. I have tried adjusting the tension…I have also tried placing a paper underneath the fabric. They all failed.
I have ordered a walking foot online so I’ve yet to try that route.
So for the meantime, I thought I’d ask here.
I would suggest iron-on soluble interfacing. That should keep the fabric stiff enough that it won't stretch while sewing (and suggested to use zig-zag stitch if using really stretchy stuff like spandex). When you're done sewing, just wash it and it'll dissolve in the water.
Are you pulling the fabric through the machine as you stitch, or are you pushing it through? I mostly sew with stiffer, woven fabrics, and have good luck gently guiding them through the machine from behind, but with a stretch fabric, it's important to make sure you're not pulling it through.
Also, does your presser foot have a pressure setting? My machine does, and I find lowering the pressure setting of the presser foot for thick and stretchy fabrics makes for a better outcome.
If you have access to an overlock machine, you'll get really good results. I've heard that people like to put woolly thread in their overlock, but I haven't tried. I like how the overlock gives me a nice stretch stitch even with regular cotton threads. The inside looks clean and professional.
If you are using a regular sewing machine, use a zigzag stitch. Play around with the width and length, but I'd start with a 2.5 in each and try to keep them the same (longer width, longer length).
I really like the walking foot. When you use that, line your fabrics up and if you need to, use pins to keep them together as you near the needle. Make sure they are feeding at the same speed (you'll notice if they aren't if one of the pieces of fabric is stretching longer than the other by the next pin).
Lower pressure setting of the pressure foot will also allow the fabric to feed "easier", without it stretching as it sews, so it'll help the two pieces of fabric feed at the same rate.
