Mask off the spot you intend to drill with painter's tape. The tape will protect the surrounding tile from scratches.
Draw a rough outline of the circle in the necessary spot using the hole saw's blade as a guide.
Fill a spray bottle with water.
Attach the hole saw bit to the drill. Turn it on according to the drill's instructions.
Wet the area with the spray bottle. As you drill, frequently spray the point of contact between the tile and the bit to keep the area moist and cool as you drill.
Drill through the marked hole. Be sure to hold the drill perfectly straight. Uneven pressure may cause the tile to crack. Use gentle pressure to push the drill through the tile. Let the drill do most of the cutting work. Stop the drill once it goes through the tile and the wall behind it.
Pull the drill out of the hole. If the drill will not come out easily, pulse it in reverse as you pull back.
Remove the tile and wall cutout from the interior of the drill bit.