It depends on what ... If the classic glazed, drill with a fork from the back, put the tile on a soft surface and drill slowly - you must not push. If necessary, soak the tile (for cooling) - but watch out for "electricity"!). If sintered, a diamond "cutter" is best, ideally with a high-speed engraving mill (it doesn't take long). If you don't have a router, you can also clamp the end mill to the drill - but it takes longer. The fact that - possibly, you will not have an 8 mm cutter does not matter, you will grind the required diameter gradually. And at greater depths, it is advisable to cool the cutter (water is enough, oil is better). The fact that it is glued to the wall is a clear disadvantage ... You write a "board" - it will - probably, spring and vibrate ... In that case, gently tap the glaze (with a hole, etc.), just to break it and the sight after it did not slip. You have to drill really gently (without pushing) and as soon as you drill a "shard", you have to use another drill bit (for wood?) To drill the board. If you have the opportunity to use the diamond cutter, definitely use it to "drill" the tile (fast and "clean", there is no risk of the glaze chipping) and only drill the base plate.