Selecting a new faucet is like choosing a new pair of shoes--you pick from plenty of styles, then you make sure it fits properly.
If you're replacing an existing faucet,
measure the distance between the
water supply pipes. Or, disconnect
the unit and take it to the store to
match up. The new faucet must fit
the holes exactly.
If on the other hand, you'll be installing
an entirely new fixture, choose the
fixture first, then buy a faucet that's
compatible. Don't worry about supply
connections; flexible plastic tubing lets
you compensate for differences between
the faucet spacing and shutoffs.
Disconnecting an old faucet can be tricky
if the old connections are corroded or
tough to get at. Shut off the water and
slip a bucket under the shutoff valves to
catch water that may remain in the pipes.
Carefully fit a wrench to the connector nuts underneath the sink (you may need a basin wrench), and make sure the wrench has a good grip before you apply pressure. If the connectors won't budge, apply penetrating oil, wait 20 minutes or so, and try again. As a last resort, heat the nuts with a flame from a propane torch, then turn them loose.
The water lines to faucets typically connect with compression fittings that thread onto their inlets and are threaded or soldered to the supply lines. If you have to soldier, dismantle the faucet's working parts so they won't be damaged by the heat.

Installing deck mounted faucets. For the distance between centers of pipes, measure from the outside edge of one to the other's inside edge.
Insert the faucet connections in the deck holes. Refer to installation instructions for particulars. To connect pipes in tight quarters, you may need a basin wrench.
It can adjust to several positions. Tighten locknuts below before connecting water supply pipes. Some copper tube connections have flared or compression fittings. Take care not to kink the tubing. If using copper tubing, bend to fit the connections, but don't kink it. Use pipe dope or tape on the threads. The faucet may have a sink pop-up. If so, install the connection on the slide rod below the unit. Lastly, turn on the valves and test the pipes and faucet for leaks. If you spot any, tighten the connections.
In A Hour.com
Installing New Kitchen Faucets