Paint your object with glaze. Keep in mind that glaze will run. Cover the bottom of your pottery with wax to shield it from sticking to the kiln shelf itself. If you want more precise lines, paint with "bisque stain" and then cover with a clear glaze.
If your surface isn't smooth, use a 100 grit sandpaper or a kitchen paring knife edge to make it so. Then sponge the entire surface of the pot to remove any dust left from sanding to provide a clean surface for the glaze to adhere to.[2]
Glazing takes on a number of forms. You can dip, brush, sponge, or etch -- just to get the list started. Use wax to keep the bottom from getting glaze on it during firing. You can also buy glazes in liquid or dry form. If you want to be a real pro, eventually you can make your own.