Porcelain is extremely hard, so it needs a specialist drill bit. Ideally use a porcelain tile drill bit, or at the very least a diamond-tipped bit. A standard masonry bit won't cut through porcelain and is likely to crack the tile.
- Put masking tape over the spot to stop the bit slipping.
- Drill slowly, without hammer action, and keep the bit cool. Many diamond tile drill bits are filled with wax, which melts as you drill and keeps the tip cool, so you can drill without water. If yours isn't wax-filled, keep the drilling point wet with a little water as you go.
- Once you're through the tile, switch to a masonry bit for the wall behind.
Where possible, drill through a grout joint instead of the tile. It's much easier, and there's no risk of cracking a tile.