Turn off the water at the cold-water shutoff valve underneath the sink.
Disconnect the water line from the shutoff valve by loosening the nut and removing the tubing from the valve fitting (Image 1). Use a basin wrench to loosen the nut holding the tubing on the faucet stem (Image 2). Remove the tubing.
Position your filter unit on the cabinet wall closest to the cold water line. Mark your location through the holes in the mounting bracket. At the marks, drive a couple of screws about halfway. The screws will be tightened completely after the filter is hung in place. Important: Leave enough clearance below the filter so you'll be able to remove the canister and change cartridges easily.
With the location marked, you're ready to make the plumbing connections. When connecting the filter, consider using flexible stainless-steel supply lines, or "flex lines," instead of the plastic tubing included in most kits. Flex lines thread onto the brass fittings and come in various lengths to match the job. They're much sturdier than plastic tubing and are available at most home centers.
Thread the brass fittings that come with the kit into each side of the filter cap. For a more reliable seal, use Teflon tape. Tighten the fittings with an adjustable wrench, but don't over-tighten (Image 1).
Thread one flex line onto the fitting marked "in" and another onto the one marked "out" (Image 2).
Connect the flex line that leads from the "out" fitting to the threaded faucet stem.
Snug the plumbing connections.
Slowly turn the water supply back on.
Release any trapped air with the pressure-relief button. Check for leaks.
Note: Make sure to replace the cartridge at recommended intervals, according to the manufacturer's instructions.