Doodling in the round is satisfying because it has no beginning or ending.
Start by drawing a circle in pencil or pen on a page of your book and ideas will begin to flow: Draw progressively smaller circles, (trace objects to get perfect circles) and start filling the rings with small, repeated shapes. Work in pencil and color them in later or start right in with colored media, markers or colored pencils. Just keep repeating one shape until you close the circle. Shade or color the spaces around each small image to unify them.
Doing doodles and drawings in a circular format is such fun because doing it takes no great skill and the rings of your mandala will almost do themselves.
Loads of fun and a good way to relax and get a dramatic drawing as a bonus.