Very slowly and gently.
I usually use a HSS bit best to get through the glaze and then change to a masonry bit for the rest - use eye protection when drilling.
Additionally, use some masking tape to place an "X" across the face of the tile. Drill the hole very slowly with a good carbide tipped bit, as woodchopper advises, with very light pressure. The tape will keep the bit from slipping and scratching the surface as well as help to keep from cracking the tile...
There is a tool that scratches into the surface of tiles thus removing the glaze(a bit like a bradawl) and then a small pilot hole can be drilled without any slippage. I have one of them but I am damned if I can remember the correct name.
Sunderland
You can buy special "Glass and Tile Bits". Use one of these first to drill through the tile and then enlarge the hole to the required size using a quality masonary bit. Take things slowly though !!
Good luck
FBG40
I was once given a question in an interview on what type of drill head was used to drill into glass. The answer was one with a large included angle. I presume the same must apply to tiles.
Something I learned a few years ago - don't try to drill ceramic tiles with a wood drill. It makes the drill very hot and it goes blunt quite quickly! - (embarrassed smile and red face)