Carefully fold all four in half at the same time. Spent time making sure the top and bottom edges line up. Rub the fold with your wooden folder to make sure you have a crisp fold. Repeat this with all your text paper so that you have 8 sections made up of 4 sheets of paper each. I can not stress enough how important it is to have as flat and crisp a fold as possible.
Book makers like to put the folded sections in a press overnight and really compress that fold.
If you have a flower press you could try pressing the sections with it. Otherwise, rub well with your wooden folder. Stack up the sections and jog them so that top, bottom and folded edges are even. To make the slits, make sure the stack of sections is jogged together evenly at top, bottom and folded edges. Lay the stack on a table with the folded edges facing you.