The next photograph, below, demonstrates the use of a cheap foam roller to which I took a pair of scissors! Just cut out gouges here and there in the foam, at random or symmetrically. Then ink up the roller evenly on a pane of glass or on a flat sheet of metal, plexi, or china. Use block printing ink, acrylic paint...even tempera will work (but it won't be permanent).
Here, I allowed each color to dry before proceeding with the next, but how about THIS for an idea: Ink your roller with two different colors, one on each side, and roll them simultaneously? Choose colors that work well together.
What an easy way to create decorative papers!