It would help if you posted a photo of the shirt you already made, but...
When attaching a separate knit trim, the length of the trim needs to be a little shorter than the edge you're sewing it to. (And I'm talking stitch lines, not raw edges.) You stretch the trim as you sew it down so it'll fit, and distribute it as evenly as you can. This will cause that edge to cinch instead of warp, because the trim is trying to contract to its original shape. The shirt fabric might look funky and rippled when it's not on you, but when you put the shirt on, it'll stretch back out.
If the knit you're using doesn't have enough rebound on its own, you could sew narrow elastic in there too.
If you have a serger with differential feed, make use of it.
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