Spread a thin layer of paint out on your palette with a palette knife, charge the roller with paint and go for it!
Step 1: Techniques for Making the Roller Designs
There are several techniques for making the rollers. You can carve into the plastic sleeve itself with a Dremel tool. You can also glue things to the surface.
I sometimes use acrylic gel, or modeling paste, for sticking things like string to the surface of the PVC sleeves. Clear PVC cement works well for sticking down shapes cut from Nagahyde upholstery vinyl. For precise gluing with the PVC cement I sometimes use a hypodermic syringe without the needle. The solvent in the cement eventually damages the syringe, but it is good for a while.
Check out the pictures for ideas as to what is possible.
Step 2: Beginner's Roller Handle
Sections of 1 1/4" PVC pipe fit on the standard paint roller handles. In this step, you see how I made a quick dremel-carved sleeve. After penciling the design on the sleeve, I held it in a vise while carving.
Step 3: Advanced Roller Handles
Inside the sleeve, there are two molded end pieces with the holes for the axle bolts, and a split ring spacer sleeve to keep them the right distance apart. These units slide into the pipe sections that are the roller sleeves.
I made the male part of the molds for the end pieces out of plywood. The female outer part of the mold was a section of the same PVC pipe meant to be used as the roller sleeves. I may have wrapped that section with wire to keep it from expanding as it was in use with heated material and might absorb heat and soften during the molding process. It was many years ago, and I threw out the molds.
For tips on working with PVC, see my other instructable: http://www.instructables.com/id/PVC-Its-Great-for-Inventions
Step 4: Paintings Done With Rollers
These are some paintings I did many years ago on water color paper with acrylic paints. For the most part, I put down abstract backgrounds with the rollers and then did something more realistic over it with brush work. A couple of the paintings were done only with rollers and paper stencils -- no brush work.