Changing or installing a new faucet is a fairly simple home improvement task, even if it does require contorting your body into a pretzel-like shape under the sink temporarily.  If you can avoid needing to be hoisted out from under the cabinet and being put into traction (I recommend stretching first) this type of project should take less than an hour.

GETTING STARTED

All sinks consist of the same basic parts:
–  Hot and cold water lines underneath the sink, usually with shut-off knobs located on each
–  Drain pipe
–  Sink basin (with anywhere from 1-4 holes–important to note when purchasing a new or replacement faucet)
–  Faucet

If you have to remove an existing faucet, it’s always a good idea to assess the situation before you start. If the sink and faucet are old or rusted it may take more time and tools to remove it.  (I always like to keep a sledgehammer nearby. You know, just in case!)

What you will need:
–  Faucet (to fit number of holes in sink) and accompanying parts
–  Teflon tape
–  Plumber’s putty
–  Wrench (a basin wrench works best for those tight spaces)

Most faucets are attached to the sink by a plastic nut—or a metal one in older models. There are also faucets that mount with bolts on the top, but more than likely you will need to wedge yourself inside the cabinet under the sink to accomplish this project.

To remove an old faucet, shut the water off at the pipes under the sink or at the main shut-off to the house. Then remove the water connections and the nuts securing the faucet to the sink.

INSTALLATION

Step 1: Adding the Gasket
Before the faucet is attached, there should be a seal between the faucet and the sink. Some faucets come with a plastic or rubber gasket. If not, you can make a snake from plumbers putty (just like you did with Play-doh as a kid), and put it on the sink where the faucet will sit.

Step 2: Positioning the Faucet to Sink
Attaching the faucet to the sink is pretty easy. Just set the faucet into the proper holes (once the gasket or putty is in place), position yourself under the sink, and screw on the plastic nut. If you’ve used plumbers putty you can clear away the excess with a spackling knife or use a finger.

Step 3: Connecting to Water Lines
Some faucets (Delta Faucet brand, for example) come with flexible PEX lines connected to the faucet already, which makes this step much easier as the hoses just need to be connected down at the water lines. For other types of faucets, you’ll need to attach flexible piping (available at hardware and plumbing supply stores, lumber yards, and home improvement centers) at your line and then to the faucet.

When attaching water lines, wrap a bit of Teflon tape around the threads to give everything a tight seal.

If you do have to attach the water lines to the faucet behind the sink basin near the top of the cabinet, it will be well worth your while to use a basin wrench. Or you can struggle through with a regular wrench. I did it and it works, but there’s always the off chance your fingers go numb and you drop a wrench on your head. Not that I would know first hand . . .

Once this step is complete you can turn the water back on, check for any leaks, and enjoy having running water in your sink!

Source: https://www.bobvila.com/articles/how-to-install-a-faucet/