What are the best practice in drilling anchor screw holes in right locations?
Level, laser or bubble. If the mirrors are all the same, and the mounting location is the same, you could use a template based on the location from the ceiling down. Mark your line horizontally on all the walls, then place the mirror on the wall where you would like it hung, then mark the edges of the mirror on both sides on the horizontal mark. Once that is done, measure in from your mark on each side, place your X, drill your hole and install your anchors. Other options would include cable hangers, saw-tooth...