Most people like to position the mirror above the vanity. Once you've chosen your position, use your pencil and level to mark out where you want the side and bottom edges of the mirror to be. In this case, we've lined up the outside edge of the mirror with the side of our vanity unit. We then set the mirror height so that it sits squarely in the middle of the tiles.
Stack some packing blocks up, level with the bottom edge you marked on the wall. To give the blocks some stability, you can stick them to the wall with masking tape.
Then remove the backing from the double-sided tape on the mirror. Rest the base of the mirror on the packing blocks and pivot it into position on the wall. Push gently against the glass, starting in the centre and working out towards the edges. This will spread the silicone and improve the bond.
Finish by sticking some tape in the corners, and give the silicone 24 hours to set properly.