How do you sharpen a porcelain knife?

When you say "porcelain" I assume you mean ceramic. Sharpening a ceramic knife, actually sharpening any knife, is not something you should attempt to do unless trained. The process of actually uses the correct angles and technique to sharpen a knife is not simple. Ceramic knives in particular are a pain, as you need very sharp materials to sharpen them (either diamond, the hardest natural compound known to man, or silicon carbide, a close second). Ceramic knives also do not develop a "burr" like other knives which tells you when you've actually created a new edge. Basically, if you want to sharpen one of these, take it to a professional sharpener or send it to the manufacturer (who will do the same).

Ceramic knife are not meant to be sharpened. Usually, if a ceramic knife becomes dull, the manufacturer would have you send it to them for replacement or return it to the store from which it was purchased.