Porcelain sinks seem to have a paradox about them: they're durable and hardy, but they crack easily at the first sign of a drill. The good news is that drilling the initial hole is the most difficult and riskiest part of drilling into a porcelain sink. You'll still need to exercise caution when expanding the hole, but it's less likely that you'll crack the porcelain after you've made the initial hole.
Size up the hole with a diamond-tipped drill bit. Find a bit that fits tight into the hole but will allow you to expand the hole once you begin drilling.
Place the bit into your drill and press the bit slightly into the hole. Do not force it; the porcelain will crack if you do.
Begin drilling at slow speed. Let the drill do all the work. Always keep the drill straight. Replace the drill bit with a slightly larger bit, increasing in tiny increments. For example, if you just drilled with a 1/4-inch drill bit, use a 1/3-inch or 1/2-inch bit. If you drill with the same drill bit for more than 15 seconds, stop and rinse the drill bit in cold water then dry it. Repeat the drilling process until you have widened the hole to the desired size.
Use a diamond-cutting hole saw if the size of the hole cannot be achieved with drill bits. Hole saws are designed to make significantly larger holes than drill bits can. Tom Sawyer of Ask The Builder advises using only diamond-cutting hole saws that are equipped with pilot holes. Only use a hole saw if you cannot achieve the desired hole size with a drill bit as you're much more likely to crack the porcelain. Apply masking tape around the hole so that the saw can better grip the porcelain.