Here's my how to:
Place a towel, piece of paper and crew neck t-shirt on a table. Use a pin to poke holes through the paper, in the shape of the neck.
If you look closely, you can see the pin holes.
Use a French Curve to draw a collar shape coming down from the pin marked neckline.
Fold paper in half (so both sides will match) and trace the shape. If you don't have a light box (I don't), just hold it on the window glass (in daylight) and trace.
I love using supplies I already own. Here is pre-quilted navy silk velvet and gunmetal gray silk shantung. Just enough for this project! Trace the pattern onto the wrong side of the fabric. Since this fabric can be slippery, I sewed before I cut the piece out. Stitch only the lower edges 1/4" seam allowance. Insert the ties (see below) and continue stitching
but be sure to leave an opening for turning.
You could use ribbon, but I used some silk shantung bias strips (2" folded in half).
Clip curves, turn right side out. Hand stitch the opening shut, tucking in the raw edges.
I think this old t-shirt could make a neat collar too, with a completely different look.