It's actually really simple, I used masking tape around his neck and drew on that what shape I wanted, then I cut that out and used it as the pattern for the collar. Once I had that done, I taped some paper between the sides of the collar and drew out the shape I wanted for the tie, then I just cut out the pieces twice, and stitched them together! Easy-Peasy.

(I also cut the collar in half so that I could attach velcro at the back, making the tie removeable)