Assuming you don't have a serger, you have two options.
First off, make sure you are using a ballpoint needle when sewing on knits (t-Shirt fabric) or you will wind up with runs in it.
1 "turn and topstitch" Take your squares and sew wrong sides together using a stretch stitch if your machine has it or a tricot stitch (takes 3 stitches each way to make a zig zag) leaving an inch or two open. Trim fairly close to the stitching. Turn right side out and using the same stitch go all the way around again with the edge of the sewing foot at the edge of the fabric.
2 with right sides together, use on of the 2 stitches above or a narrow zig zag and go around the fabric. Then use a straight stitch and go inside of the zig zag about the width of the zig zag stitch and stitch all the way around.
SonyaV
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