@MarineHaddock I'd like to recommend a website: http://www.mylifeallinoneplace.com/p/midori-tn-inserts.html?m=1

There's a bunch of printable inserts if you'd like something more like a planner, but there's just plain lined or grid paper too if that's what you want. Though YMMV depending on how much of a jerk your printer is. There is also a video on how to make inserts on that page as well.

For a first insert however, I'd suggest just printing off some grid paper from a generator and trimming it down. I like this generator personally: https://incompetech.com/graphpaper

There's plenty of options there, though it does have wide margins when it prints, at least for me.

If you don't have access to a printer, a craft shop may have gridded paper with non-reproducable blue lines. It would be most likely where the drafting/technical drawing supplies would be.

If you don't want to invest in a special stapler just yet, you can poke holes using an awl or small nail in the folded paper, and use needle and thread to bind the insert. Just like a real bookbinder! The sewing itself doesn't need to be complex either. The cover will help keep it together.

For the insert cover you can use almost anything. You can splurge and get some fancy cardstock if you like, thick-ish construction paper, or watercolor paper. The notebook cover will do most of the protecting of the paper within, so this only really matters if you're going to keep the insert for later. I do suggest that the cover be a little taller and wider than the paper, just to help cover it as much as it can.

You can use scissors or a blade and straight edge to cut down the paper, but a paper trimmer will make things a bit easier. You'll be cutting dozens of times, depending on the size of the book.

I can show pictures of the binding technique I like or my paper trimmer when I get home this evening, if that'd help you at all.

Good luck!

@MarineHaddock

I've been making my own traveller's notebook inserts for years, so if you need any help with DIY inserts, let me know and I'll pass any tips I have along!