I am currently making my son a Sketchbook shirt and I would love to share a little trick !
I found it a little tricky to get the collar to fold nicely .So , after cutting and trimming my interfacing , I cut it along what would be the cutting line of collar and stand. I then interfaced leaving a tiny space between the two pieces of interfacing.
It worked brilliantly, the collar folds nicely and stays !( I took a photo if any one is interested.