Make sure they are clean. I have never sanded a pot for any craft and they are all perfectly fine if sealed correctly. You can use any of the Acrylic arts and crafts paints from Hobby Lobby or some other craft store. Mine are mostly Americana brand but I also have Delta and a few Anita's.
Paint the designs you want then let them completely dry. Once they are dry spray them with an aerosol sealer. If you want a shiny pot use gloss if you want it to not look sealed at all use matte finish. I seal the pot inside and out, it has never hurt any plants I have put in them.
Here is a link to the Hobby Lobby Americana paints:
http://shop.hobbylobby.com/crafts/paints-and-accessories/...
and here is one to Anita's:
http://shop.hobbylobby.com/crafts/paints-and-accessories/...
This one is to their Delta paints:
http://shop.hobbylobby.com/search/default.aspx?F_All=Y&am...
And this one is to the clear sealers, but truthfully I always just by Krylon clean matte finish sealer if I want a matte finish:
http://shop.hobbylobby.com/store/default.aspx?DepartmentI...