This is really an update since a couple of people helped me with advice on this collar. I made a leotard with a stand up mandarin collar, in case anyone is interested this is what I did:
I drafted a mandarin collar as you would for woven fabric and split it at the CB and added a bit extra at the CB for seams etc. I made it with the same fabrics for lining the inside as the outside, though next time, because it is a bit bulky, I think I'd just use a stiff power net inside. The collar pieces are sewn RS to RS and under stitched along the top seam to the inside. All stitching was in ZZ.
I put the collar on to the main leotard applying a little stretch but not as much as I usually would for a full leotard collar. The I topstitched the collar seam allowance to the leotard, to help it stand up. It works nicely.
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