An under-sink reverse osmosis system works only when every connection is dry, every fitting is seated, and the storage tank holds the pressure the maker specifies. The install is simple in outline, but the mistakes tend to show up later as drips under the cabinet or a tank that never fills properly. Work through the steps in order and test before you trust the water.
Before you open anything
Confirm that the cold water line you plan to tap feeds the kitchen sink and that it has a working shutoff. If the shutoff is stiff or does not close fully, replace it or install a new one before you start, because you will need to isolate that line again later. Measure the cabinet space for the manifold, the storage tank and the drain connection. Look for a drain point that allows an air gap and a path to the drain that does not run uphill.
Check the faucet hole. Most systems need a separate faucet, and a sink with no spare hole may need a deck plate or a different mounting. Settle this before you buy fittings for the wrong layout.
The install itself
Shut off the water to the kitchen cold line and open the nearest faucet to relieve pressure. Mount the manifold and the prefilter housings first, leaving the membrane housing for later so the parts stay clean. Install the drain saddle on the cold side of the drain trap, not on a hot line, and confirm that water flows freely from the drain tube. Connect the feed tube to the manifold with the fittings the maker supplies, and push each tube fully into its fitting before you tug it to confirm the grip.
Before you connect the storage tank, check its air pressure with a gauge against the specification printed on the tank or in the manual. A tank that is too low or too high will deliver water poorly, and the system can seem faulty when the real problem is the tank. Connect the tank only after the pressure check passes.
Install the membrane and post filter as the instructions describe. Turn the supply back on slowly and watch every fitting for seepage while the tank fills. Do not rush this step. A loose fitting usually shows up within minutes as a slow drip.
Flush, test and hand over
Run the system through its flush cycle, which usually means drawing from the dedicated faucet for a set period before first use. Discard the first batch of water, since it may carry carbon fines or residue from manufacturing. Check the sink cabinet again after the tank has filled and after a few hours of normal use, and check the drain tube for slow flow that suggests a restriction.
Before you call the install finished, write down the answers to these questions: which filters get changed and on what rule, where the shutoff for the RO line is located, what the tank pressure reading was, and who to call if the system leaks or stops producing water. Keep that note with the manual. These steps are general, so follow the printed instructions for your model wherever they differ.