Purchase a Moleskine notebook or use one you already own. Measure the dimensions of a single page. Count how many pages are already in the notebook, as this may be the maximum number of leaves of paper you may replace, depending on the new paper's thickness. Remove the paper from the binding.
Decide what kind of paper you will use. Depending on your needs, 20-lb. bond paper is a standard choice for writing or sketching. A heavier paper may be needed for watercolors, which can be found in any art supply store.
Cut the paper to the size of the measurements you took in Step 1. Create a signature by pairing up two sheets and folding one inside another, creating a kind of "booklet." Fold each sheet down the middle, creasing with a spoon or rounded instrument, making sure the crease is not too sharp. Depending on the size of the Moleskine and the number of pages, you should have five to eight signatures.
Place your signatures on a piece of cardboard. Use a bookbinder's needle --- or any instrument that makes very small holes --- to poke five or six evenly spaced holes into the crease of your signatures. Make sure the holes match on all signatures.
Sew the signatures together with a waxed thread. Make the thread tension tight as to not leave slack, yet not so much as to strain your paper. Thread through the holes of one signature first. Then thread all of your signatures together to make a tight binding.
Glue the Moleskine cover to the binding of your newly secured signatures (anything from bookbinder's glue to white Elmer's glue is acceptable). Replace the newly-sewn papers by placing its binding against the cover binding, gluing the papers into the crack. You may glue a page against the back and front covers as endpages, yet this is not necessary. Place the book under a great amount of weight and allow the book to dry overnight before use.