A couple of things I learned when sewing with t-shirt type knits:
One can sew them with a straight stitch machine- the Stretch and Sew classes suggested: turn up the hem just once - pin the hem up -
LENGTHEN the stitch - stretch gently while sewing - after sewing the fabric should flatten out and there will be some built-in stretch
Practice on the cut-off piece to see how it's behaving.
If you have a zig-zag machine, you can use the zig-zag stitch.
I think one can use a double-needle to hem them, but I have never done it, so can't tell you how that would work out
You can also hand hem them - just turn up ONCE -
If you use a short stitch, the fabric just puckers and lays wonky.