Watch this video from This Old House to learn how to install an under-sink water filter.
1 Close the cold-water shut-off valve leading to the kitchen sink.
2 Disconnect the copper cold-water pipe from the supply line that runs up to the faucet.
3 Wrap Teflon tape around threads of tee fitting and adapter.
4 Tighten outlet fitting to the tee fitting using an adjustable wrench.
5 Connect the tee fitting to the threaded adapter onto copper cold-water pipe.
6 Join ¼-inch tubing to inline shut-off valve.
7 Connect tubing with inline valve to tee on cold-water line; tighten with adjustable wrench.
8 Screw filter housing to inside of cabinet.
9 Remove soap dispenser or spray hose from sink deck; if necessary, drill new hole in sink deck.
10 Place bead of plumber's putty under faucet.
11 Set faucet in hole in sink deck.
12 Use basin wrench to tighten faucet nut from below.
13 Connect tubing to faucet stem with compression fitting; tighten with basin wrench.
14 Connect cold-water tubing to inlet side of filter housing.
15 Connect cold-water tubing to outlet side of filter housing.
16 Turn on the cold-water shut-off valve, and check all fittings for leaks.
17 Test the water-filter faucet.