Mirror seems smaller than it is (120cm×90cm)

Step 1: What U Need

-Accu drill
-Dowels 6mm for drywalls
-Screws
(length of mounting bracket + dowel length + some mm to catch the end of the dowel)
-diamond drillbits (45$ / set)
-standard drillbit 0.5mm smaller than diamond drillbit
-drill gauge with water connection (10$)
-prechargeable water bottle (10$)
-masking tape (not necessary)
-measurement tools
-pencil

Step 2: After Measuring and Marking

I don't want water and dust to go between washing basin and wall (there is no sealer jet)

Step 3: Install the Drill Guide - Drilling

i used a 6mm diamond drillbit
the guide is fixed by suction cup.
push the trigger of water bottle.
so that water flows permanent out of the guide. Not much but enough to hold the drill edge cool.
Then start drilling.
Depending on tile hardness, it needs up to 2 minutes for about 9mm tiles.
Relative high rpm and medium pressure.

Step 4: Result in Clean Holes

Step 5: Drill Trough Drywall

stop with diamond drillbit immediately when u are through tiles.
drilling with that kind of drillbit will result in stump edge.
so u will need a normal wood or Metall drillbit for this drywall plates.

I used a 5.5mm drillbit for that.

Step 6: Dowels

Insert Dowels and mount the brackets, mine where shipped with mirror, and they are adjustable.
in case of lag of measurement.

Step 7: Finish

I haven't described the electrical connection.

my conclusion...
it's better to use tools which are made for such kind of job, than get broken tiles.
I tried this before on a piece of tile which was over.