First time crocheter. What am I doing wrong on this project?

Beginner crocheter. Decided to do a mindless project to get me ready to follow eventually more difficult patterns. So I am following a Chat GPT pattern to make a book sleeve.

Essentially, it’s chain length of book. HDC into 2nd loop to end. Chain 1, turn. Rows repeat till you’re able to fold in half to cover book.

Except, somewhere, it started to expand. I don’t recall adding or doubling up in loops so I’m not sure why it’s fanning outwards causing it when folded to not line up.

Any suggestions or feedback? Planning to frog and redo. (I’m a perfectionist and this is a b-day gift for a friend I’ll be seeing in 2 weeks)

Thanks!

I’m planning to add a rim with sl st and creating a loop with button.

The sides say to sew the edges together. Could I do that by sl st?

Thanks!