I am assuming you are talking about covering books with a spiral binding, not a composition book. I LOVE doing these and they are very easy. I just cut my paper the exact same size as the book (for a notebook, I just pull out a piece of the paper... but you could actually measure). Then I use that piece of paper that you took out and lay it on the top of your cut CS. Then use a pencil and mark off the center of each hole. Punch down the line of holes, centering the pencil mark. Then snip your paper from the edge of your CS to the hole, that's right, just snip. Don't tear anything out. Lay the CS with your holes on top of the spiral binding and gently push down with the eraser end of your pencil (or the end of a paint brush) and gently slide the slit over each of the spirals. Viola! You now have a new cover that you can embellish to your hearts content.
The beauty is that you can cover a notebook of any size this way. Here is one I did if you are visual:
Rosey Posey Planner
This notebook just happened to have an opaque cover, so I left it on, but they work just fine without a cover.