The first step is to measure your vanity mirror length and width.
How to frame a bathroom mirror on the wall.
Measure the thickness of the mirror and the bottom track if any.
Determine trim style and positioning of trim around mirror to obtain measurements specific to.
Make cuts for all four sides.
For example say your mirror is 64 long and you chose a 4 square block for the corners.
Once your frame dimensions are known cut the four sides 1 longer than that final size.
In order for the frame to mount flush against the wall a notch must be cut into the backside of the frame pieces.
Press onto the mirror or wall and use a level to make sure the frame is.
Pick your block and subtract the size of each block from the size of your mirror measurements.
How to frame a mirror.
Measure mirror and select frame style.
Using your miter saw cut your wood at a 45 degree angle so the outside portion of your wood piece will be the full length of the mirror see pic above.