An under-sink reverse osmosis system connects to the cold water line, a drain, and a faucet at the counter, with a storage tank and filter housings tucked into the cabinet. The steps below are the usual order. The manual that came with your kit overrides this outline wherever they differ, and local plumbing rules may require a licensed plumber for some of the work.
Before you open anything
Read the manual through once. Check the parts against the packing list so you know what is missing before you are lying under the sink with the supply off. Measure the space for the tank and housings, confirm where the cold line runs, and check whether the sink has a spare hole for a new faucet. Pick a drain connection point above the trap. Gather a tubing cutter, an adjustable wrench, and a towel.
Shut off and relieve the line
Close the cold supply valve under the sink. Open the kitchen faucet to drain the line, and keep a bucket underneath for the water that comes out when you disconnect fittings.
Mount the faucet and the drain
If the kit needs a new faucet hole, this is the first point where a plumber may be needed, since drilling a countertop and fitting a new faucet can involve work that varies from sink to sink. Mount the drain saddle or adapter on the drain line above the trap, following the manual. Route the drain tube so it does not sit in standing water.
Connect the feed, the housings and the tank
Install the feed valve on the cold line. Cut the tubing square, push it fully into each compression fitting, and tighten the nut until snug without forcing it. Connect the housings in the order the manual shows, then the storage tank and the faucet. Keep the tank valve closed until the system has been flushed.
Restore water slowly and check for leaks
Open the cold supply slowly. Watch every fitting for drips with a dry paper towel. Tighten only the joints that weep, a little at a time, and recheck after the water has run. Then follow the manual's flush and startup steps, which usually means running the faucet for the stated time with the tank valve closed, then opening the tank valve and letting the tank fill.
Where to stop and call a plumber
Stop when the job calls for soldered copper, a new trap, a permit, or any work you have never done before. Also stop if you find a leak at a fitting you cannot reach, or if the cold line is corroded or damaged. A short professional visit to finish a difficult connection is usually easier to manage than repairing water damage under a cabinet.