A simple, but very easy technique. Try this with colored paper.
STEP 1 - Sketch and envision and then lay down a layer of oil pastel. The paper is as important as the pastel you are using, so choose one that lends itself to your vision.
STEP 2 - Choose a liquid medium like ink, watercolor or gouache and apply using a paintbrush atop the layer of oil pastel. The paper will absorb the water and the pigment.