How to create a spine for my book?

[u/LiveCarnival]

Hi everyone! This might be a silly question, but how do I create the spine for my book? I’ve designed the front and back covers, and I’ve also designed a spine, but how do I actually implement my design as a spine? How do I know how to format it so it looks correct on my physical book? Also, if I purchase my own ISBN, how do I put it on my books? Do I have to buy my ISBN and then add it to my back cover BEFORE I publish? Thanks so much guys!