Years ago I read a library book about art and in particular composition. The only part that I clearly remember is author recommending taking a business card or credit card to trace a bunch of little rectangles onto a page of your sketchbook. (she maintained that this was the only legitimate use for a credit card and I think she may have been right) Then you scribble very tiny shapes into those little spaces. The theory was in part that it frees you from getting hung up on detail and just work on the mass shapes. In fact you often just draw shapes prior to rationalizing what the shapes are. If the composition wants a diagonal line you draw a diagonal line and then work out what that line represents later. A sword, a lance, the edge of a rocky cliff behind the character, a cape blowing in the wind, a cloud-bank. Just draw it first and then keep noodling the sketch until things begin to emerge. And if nothing emerges do another one you have plenty of paper. The whole idea is to generate ideas.
These are called "Thumbnail Sketches" and it is vital that you get comfortable doing them whether you are painting landsacpes, still lifes, wildlife, or book illustrations. Even when drawing on location doing a variant of this exercise helps you to narrow down which elements of the scenery you're going to focus on.
On the left center row you can see the thumbnail sketch for the drawing above and for an upcoming entry.