Replacing a vanity or vanity top is one way to change the look of your bathroom. Vanity tops are made from a wide variety of materials. Some preparation work is necessary before you install the bathroom vanity top. You must ensure the vanity is ready to accept the vanity top. It is also a good idea to install the plumbing hardware on the vanity top before installing it on the vanity. This gives you plenty of room to tighten the securing nuts to the faucet and sink hardware, rather than working in the tight confines under the cabinet later.

1. Place a level across the top front of the vanity frame to ensure the vanity is level. If necessary, place shims under the front of the vanity until it is level. Move the level to the side of the vanity, and repeat the leveling process. If necessary, remove screws attaching the vanity to the wall, and level the vanity. Reattach the screws through the vanity and into the wall to secure the unit once level.

2. Assemble the faucet you are using onto the sink portion of the vanity according to the directions from the faucet manufacturer. Remember to apply a bead of plumber's putty on the bottom of the sink drain lip before installing it into the sink. The putty helps prevent water leaks. Test the operation of your faucet stopper lever before installing the vanity top, to ensure all parts are working correctly.

3. Position the vanity top on top of the vanity frame. Depending upon the size of the vanity top, you might need a friend help lift it on and off the vanity. Ensure the vanity top sits plumb on top of the vanity. Look under the vanity where the frame makes contact with the vanity top to ensure there is contact all around. Dry fitting in this manner helps you find potential problems such as raised nails, before applying the sealant. 

4. Lift the vanity top off the vanity. Apply a 1/2-inch bead of silicone caulk on the top of the vanity frame with a caulk gun. Apply silicone caulk to the back of the vanity top backsplash. When applying the caulk to the backsplash, move in up and down within 1/2 inch of the top and bottom edges of the backsplash in a "U" formation. This helps to ensure a firm contact with the wall.

5. Ask your helper to assist lifting the vanity top on top of the vanity. Position the vanity top over the vanity and gently lower it into place. Ensure the backsplash is against the wall. Press down on top of the vanity top to ensure contact across the vanity frame.

6. Attach the sink drain plumbing to the sink drain, according to the manufacturer's instructions. Apply a 1/8-inch bead of caulk between the vanity top and the wall with a caulk gun.