It depends what is causing the wave in the first place.
Use a stretch or ball point needle
Reduce foot pressure. To much pressure cause the fabric to stretch making it wavy(not all machines have this dial, but I use mine all the time)
Try not to pull the fabric as you feed it through.
Press the hem before stitching and have the right side facing up as you stitch
Use a coverstitch machine or a twin needle if you have otherwise a triple zig zag stitch or stretch stitch does the job ok
I do not use WSS on shortening hems as I do this for my day job and we can't sell something that has been washed.
Thanks for the tips!
I do what Jenne does when I don't use a Cover Stitch to hem on my Serger... Use a stretch stitch as Chris has suggested and the WSS underneath...and a ball point or stretch needle.
I use my walking foot.
You need to ensure you use a stretch stitch specifically for stretch fabrics. You should have at least 1 or 2 of them on today’s modern machines. Good luck. If you have any problems check out YouTube on “How to sew stretch fabrics”. Love Chris ❤️👵🏻