Before installing your new bathroom vanity, you will need to remove the old one and prepare the space.

First of all, as with any job involving plumbing, make sure the water supply is switched off before you start. You'll be able to do this either by turning off the hot and cold water valves inside the old vanity unit, or by turning off the mains supply for the whole house, depending on the design of your particular unit.

Then you'll need to use an adjustable wrench to disconnect the supply tubes for both the hot and cold-water valves. Next use some adjustable pliers to remove the sink trap – remembering to put a container underneath before you start to catch any drips. When doing this you may want to wrap some cloth or tape around the jaws of the pliers to prevent chrome fittings becoming scratched.

Once you've done this, you're ready to unscrew the screws holding your vanity to the wall, using a screwdriver or socket wrench. You should be able to find the screws either in braces in the back corners or in a rail along the back.

The next step is to take off the vanity top or sink to make the unit lighter and easier to move. You'll need to be careful here if you plan to reuse your vanity top. With most bathroom vanities, the countertop is attached with adhesive caulk and can be removed using a 14" pry bar. You can also use a pry bar to separate the backsplash from the wall. Be sure to take care when doing this to prevent damaging the wall. You could insert a wood shim behind the bar to help protect the wall. If the sink is in a cutout, it can be lifted out once the tabs that hold it in place have been removed. Before moving your vanity, take out any removable drawers or doors to lessen the weight. If it is possible to, you can then slide the vanity unit out. However, check whether the floor butts against the bottom of the vanity, in which case you'll need to use the pry bar to lift up the front and then slip some wood shims underneath so you can slide the vanity out without damaging the floor.

Once you've managed to remove the old vanity, it's important to inspect the space for signs of damage and to rectify any problems before fitting your new vanity unit. Replace damaged flooring or use plywood to replace any rotten sections of floor. Check the drywall for water damage and repair it if needed. Finally, using a spirit level, check the floor surface is level, in both directions. If it's uneven, it can be corrected by using shims underneath the vanity, or making a base out of plywood that can be nailed in place, and which can be leveled with shims.

Once the prep work has been completed you will be ready to begin the installation of the vanity unit. Find the studs in your walls and identify them with pencil marks so you can see where to screw the new vanity to the wall. Following this, lightly mark on the wall exactly where the vanity will go. Then find the vertical and horizontal centerlines of your pipes and mark these on the wall, taking care that they are plumb and level. Measure from the center of the drain line to the mark where the nearest cabinet edge will be, and to the floor. Then measure from the reference lines to the center of each supply pipe. These measurements will enable you to draw the pipe locations onto the back of the vanity.

Using an electric drill, make pilot holes at the layout marks on the back of your vanity, so you can make holes for your water and drain lines. Use these holes to line up your hole saw. Use a hole saw that's at least 0.5in larger than the widest part of your water and drain lines, and drill halfway from the outside and halfway from the inside of your cabinet for a smoother finish.

You're then ready to attach your vanity to the wall. Drill pilot holes in the back of your vanity that align with your wall studs. Make the holes in the vanity are one size larger than your 2.5in screws, and the holes in the studs one or two sizes smaller, for a really secure fit.

Once you're screwed your vanity in place, you can install your sink. If you've got a sink built into your vanity top, lay it somewhere you can access it easily, such as on the old vanity, then fit the faucets according to the instructions and tighten up using pliers. Attach flexible water supply lines to the faucets. Join the tailpiece to the sink and seal around it with a bead of silicone caulk. Check that the sink pop-up connection faces the back of the sink before tightening the locknut to hold the tailpiece in place. Then smooth silicone caulk along the edges of the vanity unit and backsplash and position the vanity top on the unit, pushing it side to side to center it and making sure the backsplash is tight against the wall. If you're installing a separate sink, you'll need to fit the countertop first, then install the faucets in the sink and last of all fit the sink to the countertop according to the instructions.

The final stage is to connect the plumbing and tidy up the edges. Use pliers to attach the supply lines to the water supply valves, connect the sink trap to the drain line and tailpiece, adding the pop-up control rod at the end. Run a bead of clear silicone caulk to seal the gap where the backsplash meets the wall. You can do the same to fill in small gaps if the vanity doesn't fit completely flush against the back wall, or you can nail flexible molding that matches the vanity to disguise the gaps and give a smart finish.