Well, you draw in it of course!
You can write, doodle, collage, all sorts of things. Some people use sketchbooks as a ‘Visual diary’
There is no doubt that sketching is an activity that helps you work through any design problem or any other type of problem for that matter. From the initial concept to working out the flaws, to the final design sketching can be used to save time and energy. The image in a sketch can be totally rough or even incomplete to a viewer, but to the artist, each mark, line and denoted area contains meaning and is understood fully in the artist’s mind.
Please don’t spend an hour on a drawing if it is your first idea for a project. Through sketching out your ideas quickly your mind will be able to see better options or flaws and then fix them in the next sketch. Which will lead you to see more flaws or get better ideas for the projects. In the past, I have spent whole sketchbooks working out ideas for a single painting and the parts within those painting.
Billiard room painting example:
Articles for you to read:
Here are some ‘Flip through’ videos of other peoples sketchbooks to inspire you:
Here is a short video of a guy, named Peter
And a longer video by Peter