Locate the hot and cold shut-off valves under the sink - they're the oval-handled valves on the supply lines coming out of the wall or floor. Turn both clockwise until they stop completely. Open the faucet to release any remaining pressure and verify the water is off; the flow should stop within a few seconds.

Place a bucket under the P-trap to catch residual water. Lay painter's tape along the floor and any adjacent tile you want to protect from tool scratches or dropped screws. Remove items from inside the vanity cabinet, the countertop, and the surrounding area to give yourself a clear workspace.

Loosen the slip nuts on the P-trap by hand or with pliers - they're the large nuts where the drain pipe connects to the curved P-shaped section. Slide the P-trap off and let the water drain into the bucket. Next, use an adjustable wrench to disconnect both water supply lines from the shut-off valves. Have a rag ready - residual water will drip from the lines.

Run a utility knife along any caulk lines where the countertop or vanity back meets the wall. Work slowly to avoid gouging the wall or tile. Once the caulk seal is broken, look for screws securing the vanity cabinet to the wall studs - typically 2-4 screws through a mounting rail at the back of the cabinet. Remove them with a drill or screwdriver.

If the countertop is separate from the cabinet, have a helper assist as you lift it off - stone and engineered stone tops are 50-100+ pounds and awkward to manage alone. Slide the cabinet out of its position. Scrape any remaining caulk or adhesive from the wall and floor with a putty knife. Now is the right time to patch any drywall damage, paint the wall, or address any tile work before the new vanity goes in - it's nearly impossible to do cleanly once the cabinet is in position.

Before sliding the new cabinet into position for the first time, do the prep work that's much harder to do once it's in place. Remove the cabinet doors and any drawers to reduce weight and improve access. Mark the stud locations on the wall using your stud finder - use a pencil so the marks are visible once the vanity is pushed back against the wall.

Hold the new vanity in its approximate position and mark on the back panel where the plumbing supply lines and drain stub-out will exit the cabinet. Remove the vanity, then use a hole saw or jigsaw to cut the plumbing access holes slightly oversized (1-2 inches larger than the pipe diameter) to allow for any alignment adjustments during final positioning.

If the countertop is separate from the cabinet, dry-fit it first without any adhesive to confirm alignment and overhang. The countertop should overhang the cabinet face by approximately 1 inch on the front and sides. Check that the top sits flat without rocking at any corner - if it rocks, identify the high point and address it before applying any adhesive.

Once fit is confirmed, apply a continuous bead of silicone caulk along the top edge of the cabinet (per the vanity manufacturer's instructions - some specify construction adhesive instead of or alongside silicone). Carefully lower the countertop into position with a helper and press firmly. Wipe any squeeze-out immediately with a damp cloth.

For the sink: if using plumber's putty around the drain flange, roll a rope of putty and press it around the underside of the drain flange lip before inserting it into the drain hole. If the sink manufacturer specifies silicone instead of putty (common with granite composite sinks), follow their instructions - putty can stain some sink materials. Thread the drain locknut onto the drain from underneath and tighten by hand until snug. Do not use a wrench on plastic drain bodies - hand-tight plus a quarter-turn is sufficient.

This is the step most DIYers rush or skip - and it's the one that prevents caulking over a leak that then causes water damage for months before anyone notices. Turn the shut-off valves slowly counterclockwise to open - no more than a quarter-turn every few seconds. Watch all connections as pressure builds. Check the supply line connections at the valves and at the faucet tailpieces. Check all P-trap slip nut connections. Check around the drain flange where it meets the sink basin.

Run the faucet for a full 2 minutes. Watch underneath with a flashlight - look for any dripping, droplets forming on connections, or water collecting on the cabinet floor. Dry paper towel placed under each connection will reveal slow seepage that isn't visible as a drip. Address any leaks before proceeding: supply line connections simply need another quarter-turn; P-trap connections may need to be disassembled and re-taped.

With leaks confirmed zero, apply a neat bead of mold-resistant silicone caulk along the backsplash joint where the countertop meets the wall, and along the base where the cabinet meets the floor. Use a caulk gun for consistent bead pressure - cut the nozzle at a 45-degree angle for a clean application. Apply painter's tape on both sides of the joint for razor-clean caulk lines before applying.

Smooth the caulk immediately with a damp finger or a caulk-smoothing tool, pulling in one continuous stroke along the joint. Remove the painter's tape while the caulk is still wet - peel at a 45-degree angle away from the joint. Allow the caulk to cure per the product instructions before the bathroom is used: most bathroom silicones are shower-ready after 24 hours.

Reinstall the cabinet doors and drawers. Adjust hinges and drawer slides using the adjustment screws (most modern European cabinet hinges allow height, depth, and side-to-side adjustment with a single Phillips screwdriver). Reinstall handles or knobs. Add any base trim if a gap between the cabinet and floor is visible. Wipe down the entire installation with a clean damp cloth to remove fingerprints, dust, and any silicone residue. Step back and inspect all joints in the natural light.