I use 8 pieces of A4 folded in half and stapled. You need a long-necked stapler, (or cotton and thread I suppose) and you're done. It always lies flat. A4 is super cheap. You can print covers or different types of page if you're interested in that kind of thing.
Author of the post here, I looked into that but my handwriting is so bad that I needed to have lines on the paper. It was cheaper to use crappy premade notebooks from Amazon.
