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Find a clean dish sponge (not a scour pad) and a ballpoint pen with a removable tip.  A cheap, plastic pen works best, as long as you can easily pull off the plastic tip and remove the ink. A slightly see-through pen can make it easier for you to see what you are doing. Use your finger to feather and break up the fibers of the sponge.
Gather your materials. Leave about 1/4-1/8 of an inch of the sponge sticking out of the pen.