Measure the height and width of the mirror with a tape measure.
Mark the lowest right corner of the mirror on the wall with a pencil. Do not use an ink-based pen or marker to mark the wall, as they will leave permanent marks.
Measure over the mirror length plus 1/16-inch to the left of the right corner mark and place a pencil mark. Repeat the process with the mirror height. Extend all the marks to draw a square or rectangle on the wall. Place one pencil mark one inch away from each corner and then every 12 inches until you reach the opposite corner.
Align the inside edge of a mirror clip with one of the drawn lines with the top of the clip aligned with one of the marks you made one inch in from one corner. Install a hollow wall anchor at the location if no stud sits behind the aligned clip or run a 1-1/2-inch drywall screw through the clip hole with a screw gun.
Install the remaining clips along the bottom line of the mirror, then the top and one side of the mirror. Slide the mirror in the installed clips. Install the clips on the last side of the mirror.