Determine the exact location of the mirror according to your taste, and mark it on the wall with a pencil. Measure the height of the mirror to within 1/16 inch with the tape measure.
Place the stud finder against the wall and depress the find button. Move the stud finder horizontally to locate the studs. Mark the center of each stud on the wall.
Place the level on the wall where the bottom of the mirror will be and mark a line. Set the J-channel against the wall with the bottom on the line and drill a hole through the J-channel and into each stud. Screw the J-channel to the studs with the screws.
Measure up the wall from the top of the outside lip of the J-channel the height of the mirror plus 1/8 inch, and mark the wall. Make a second level line across the wall with the level. Hold the top of the second piece of J-channel against the line; drill holes through the J-channel into the studs. Mount the J-channel to wall with screws.
Check the mirror fit in the J-channel. Insert the top of the mirror into the top J-channel, and then set the bottom of the mirror into the bottom J-channel. It should fit and still have the top and bottom of the mirror enclosed by the J-channel.
Remove the mirror from the J-channel and lay it down on a flat surface with the back of the mirror facing up. Place a 2-inch-wide circle of mirror adhesive for every square foot of mirror space. Keep the adhesive at least 4 to 6 inches from the mirror edges.
Insert the top edge of the mirror into the top J-channel. Align the edges of the mirror with the edges of the J-channel. Fit the bottom edge of the mirror into the bottom J-channel. Keep the edges aligned. Press the mirror firmly against the wall to spread and embed the adhesive.