It’s mostly the same as folding blank sheets except that you match the text edges instead of the paper edges. If your paper is thin you may be able to see through it on the bench; otherwise, hold it up to the light and pinch the edge of the fold to get it in the right spot. Assuming you’ll be trimming all three edges at the end, the priority here is for the text to line up and for the margins to be equal on all pages. If you’re lucky, the paper may line up at the edges as well, but chances are it’ll be a little bit off, especially if you’re folding up hand-printed sheets.
The only other thing that’s different is it matters which direction the folding happens. If you fold the first fold in the wrong direction, the pages will all be in the wrong order. Imposition (the printing of multiple pages on one sheet) will be different depending on the size of the book and the way you decided to lay it out. We can talk about that later. For now just notice that there’s an order of operations.
Here’s my example sheet, the first signature of a octavo. The page printed at bottom right of the recto is page 1 of the signature; the one at bottom left is the last. I made the fold before taking these photos to show which direction the fold will happen.
Some of this is redundant if you’ve read the post about blank sheets, but it’s worth a refresher:
Don’t forget to use the flat side of the bone or PTFE folder!
Once you get to the point where there’s enough text that looking at it in front of a light doesn’t really help, just kind of peek in the middle as you make the fold and be sure the text is still lining up. I suppose I ought to say that you will find at some point that no matter what you do you can’t get it to line up, because the printing is off in some way; in that case, either keep one margin consistent or split the difference and make it off the same in all directions—whatever’s the least offensive.
I purposefully left these margins a little wide on one side as a dramatic example of keeping the text lined up at the expense of the paper.
You won’t be able to turn the pages until you trim or slit the folds at head and fore-edge. I’d do a test one first to make sure you’ve gotten it all correct before you do your whole book!