Sit with your art materials: pencils and paper work well. I like to do mandalas in this way, so I draw a 4 or 5 inch circle with a compass. I then close my eyes and ask to see what to do next. I will perhaps get another picture of a line in the circle or a shape in the center, and I will draw it in the circle as I saw it. I then close my eyes and ask what to draw next, and do that- messages I receive as pictures, you may receive them in some other way... I repeat this process until I see or feel that it is complete. I sometimes ask for a title of the resulting piece, sometimes not. If what you create is awkward or ugly, that's ok, just keep doing it, until you feel like the process is serving you and you are connecting to that creative reservoir within yourself. You won't know what you can create unless you try!
This is the process I used to create the mandalas at the chapter heads in the book Be The Second Coming. I did some of them with the specific chapter in mind, and some I just did and realized afterwards how perfect they were for a certain chapter.