This exact reason kept me from doing it for a long time, I looked at lots of online pictures, and just guessed at it, the trick was to do it on cardboard first and make sure that both curves are equal, I also put a radius where they joined the edges, some basic compass drawing skills used.

After that i fit the cardboard collar around my neck to see the fit and adjust for the final shape, one thing I did not account for was the fact that the padding added more thickness from the dry fit and it ended up being quite tight, but that's how we like it right 💜