Step 2: Mark Number
I did this by spreading the pages out first (by approx double) so that it gives a better idea of what the final number would look like. You can see that when the book is not spread, the number is very tall and narrow, this is how it should be, as the folding of pages will thicken it.
When spreading the pages, spread them from left to right (as per the photo with the blank pages) then draw the pencil marks from left to right also. This way the pencil marks will be on the front of each page so that you can see them when folding.
When marking the number, try to get a fine definitive line. I used a mechanical pencil to help with this. It will mean that you have a definitive point to fold, which will help the number edges to be much more even and precise with a better 'flow'.
I did mine by free hand, but you could use photoshop or something to change the aspect ratio, print it out, and then trace it on to the edge of the book. If you were doing a word or picture or something more complex, than this might be the way to go.