use teflon tape for more reliable seal
flex lines threaded to in and out fittings
Make the Plumbing Connections
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).