I have traditionally preferred working with 5mm grid paper, but am looking into dotted grid paper.
In the past I’ve taken notes/sketched in A5 unruled spiral notebooks, which are cheap and easy to remove pages. I’ve also used spiral grid notebooks, although I’ve only come across these in A4.
I’m interested in picking up a MIDORI brand notebook.
Stack ~10 centre-creased sheets of paper and either staple or sew down the middle. This will make a 40 page bundle (since each sheet will be halfed).
Then you can sew 40 page bundles together to make a bigger book.
A slip on cover can be made from some light card, however you’ll want to go up a size from your sheets, since you’ll want to slip it into the first and last pages.
To make an ~A5 size notebook you’ll want to fold A4 pages in half.