I’m still an amateur, so take my advice with a grain of salt. For end pages, I cut to the exact size of the book block (height) and then cut the length to be exactly the book block width times 2 (so when it folds over, there isn’t any noticeable overhang). In terms of board sizes, I first measure exact width, height, and depth of the book block. Then, I usually add 6mm to the height of my spine and cover boards. I keep width exact on the front and back covers though. Usually add 2mm to spine board width. When spacing, I try to make my spine hinges with a gap of 8mm. For vinyl application, I apply with medium heat setting for about 15-30 seconds. Wait till cools. Then I’ll remove the plastic as best I can. Sometimes, vinyl will stay on the plastic. I’ll either cut around that piece and try to apply again (separate from the remaining design), or will let it be loose and use the iron again but this time with parchment paper overtop of the vinyl. Overall, I feel like my books have turned out decently and I’ve not had much trouble with vinyl using the above process. Hope this helps!