This is how the final notebook looks like. You can see in the pictures that it actually lays flat when open.
This is my process to make a sketchbook with simple materials and tools you may certainly have at home. A nice feature of this notebook is that it can lay flat on a table and its covers are wrapped in cloth.
Step 1: Folding and cutting
Gather coloured sheets of paper (A4 size) and fold once. When pressing, use a blunt object, e.g. a fork, to obtain neat folded edges. I have folded 4 sets or sections of A4 sheets (each set has 4 folded sheets). When you have folded all sets, cut in half using a knife. The final sketchbook will be approximately 10cm x 14cm (almost A6 size, 4×6in), after trimming the edges.