Sand the chipped area lightly with a medium-grit emery cloth. Sand the area smooth so that there are no rough edges. Sand only the area of the chip and not the surrounding area.
Clean the chipped area with a cloth dipped in alcohol. This will remove the dust left by the sanding and also prepare the area for the patch. Allow the chipped area to dry completely.
Squeeze a small amount of porcelain repair compound on a piece of glass, ceramic tile or metal. Add a small amount of high-gloss enamel paint that matches the color of the toilet to the compound. Mix the paint and compound together with a wooden dowel.
Scoop up some of the mixed compound onto a single-edge razor blade. Scrape the compound into the chipped area with the razor blade. Continue adding the compound to the chipped area until it overlaps the chipped edges. Scrape away the excess with the razor blade until the compound is flush with the surrounding surface.
Allow the patch area to dry overnight. Dip a cotton swab into fingernail polish remover and apply the remover to the patch. This will help remove excess compound and also blend the edge in with the surrounding area.