Locate the Wall Studs
Find at least one wall stud on the wall behind the vanity location using an electronic stud finder. Make sure to mark the center of the object. Consider finding a second stud if possible. If the vanity is enclosed by a sidewall, then find the wall studs inside the sidewall.
Set and Fasten the Vanity
Measure the height of the vanity cabinet, and transfer this to the back wall, if applicable. A level allows you to draw a straight line at the height of the object.
Put the vanity cabinet in its place. Using the line you drew, ensure that the cabinet is level both side-to-side and front-to-back. As necessary, level the cabinet using wood shims. Attach the cabinet to the back wall only using two 3-inch cabinet screws driven through the mounting strip in the back and to each wall stud in the rear. You will attach a wall attaching a sidewall after fitting a filler strip.
Fill the Side Gap
If there is a wide or unwelcome gap between the cabinet and the sidewall, you can cut a narrow filler strip to fill the gap. Filler strips may need to be bent to fit around a baseboard or tile piece at the bottom of the cabinet.
Using a small profile gauge or carpenter's compass, you can copy the profile of the side wall onto the wood strip. Create a paper template that matches the profile and trace the profile on the filler strip by tracing it with a pencil.
Take a coping saw or jigsaw to cut out the profile. Fit the filler strip and adjust if necessary. Finish the wood strip according to the base cabinet. Take a screwdriver through the inside edge of the cabinet face frame to secure the strip to the cabinet. If the strip is thin, glue it in place with a small amount of epoxy rather than screws.
Fasten to the Sidewall
Although this step is often overlooked, it is best to anchor the side of the vanity cabinet to the sidewall, as well as secure the cabinet to its back wall. Since there is a gap between the side of the cabinet and the wall for the vanity countertop, you'll need to add blocks to the gap to support the table before driving screws.
Two wood spacer blocks are made to a width equal to 1/8 inch short of the distance between the vanity side and the wall. A wood shim can be placed between the wood spacer blocks and the wall.
You should use a drywall screw that is long enough to extend at least 1 inch into the wall at a certain location. The side of the cabinet will be drawn snugly against your spacing blocks and shims.
Connect the Plumbing
Following the manufacturer's instructions, connect the water supply lines to the faucet and the sink drain to the drain trap and branch drain.
Install the Backsplash
If you have a separate backsplash, apply a bead of construction adhesive in a serpentine pattern to the back of the backsplash and push it firmly against the wall.
Caulk the Joints
You can apply silicone caulk where the countertop meets the wall. Caulk along all joints where the backsplash meets the wall and where it meets the countertop if there is a backsplash. Before using the vanity, let the caulk cure for 24 hours.