From imperial sized 56x76cm sheet of watercolour paper to about 13x13cm square sketchbook. You'll get a sketchbook of 24 sheets of paper. or if the pages were numbered, 48 pages.
First, cut along thick pink lines in order to separate sheet into 6 sections. Then cut those in half length-wise, along the thin orange line. Then you will get 12 sheets of 12.65 x 28cm paper. Fold these in half along the black dotted line.
You should have something that looks like the pic below.
The pic above is a cross-section diagram of the binding. The green line will be the cover (we'll get to that later). I used normal printer paper/cartridge paper for the orange lines. The blue lines resemble the folded sheets of paper we just made in the step above. The little black lines resemble the stitching thread marks.
Cartridge paper.
Do this step so that our covers will have something to stick to that is not the watercolour paper. WC paper is expensive!
Cut excess.
I followed this youtube tutorial on kettle stitch binding. https://www.youtube.com/watch?v=9O4kFTOEh6k
After that, I found some google images of the type of Arches paper I am using for this sketchbook. I printed it onto a thicker A4 paper so that it will be sturdy as the cover. You can use construction paper for this step. It just so happened that A4 was perfect size for length. However, we need to trim the top and bottom. No need to be perfect in measuring, just get your stitched pages and gauge how much to cut off.
Because the book as some thickness, make two creases for the binding part. Then put double-sided tape (or strong adhesive) onto the cartridge papers front and back covers. Peel the paper off the tape and stick our cover on.
Voila!
Feel free to contact me on instagram if you'd like more guidance. :)