Sink fixtures come with everything you need to attach them securely into your sink, except instructions for tightening those hard-to-reach nuts. If you are putting in a new sink, you can put in the fixture until you put in the sink, while the nut remains readily accessible. Once the sink is, however, you normally have to crawl under it to tighten the nuts, and there is seldom room to get a wrench or a pair of adjustable pliers. Do what plumbers do and use a basin wrench to make the task all but effortless.
Clear off the shelves under the sink to give yourself more room to function. If the sink P-trap is in the way, remove it by unscrewing the nuts holding it into the sink tailpiece and wall drain, using adjustable pliers.
Insert the fixture into the right hole in the sink, middle it and align it with the back wall. Have a helper hold it steady while you fasten the retaining nut out from under the sink.
Wear a headlamp and place your head so that you can see the nut readily. Screw the nut and tighten it as much as possible by hand.
Use a basin wrench to tighten the nut the remainder of the way. A basin wrench includes jaws that clamp on the nut along with a long handle so that you can reach it easily. Fit the jaws of the wrench across the nut and rotate the grip. The jaws automatically tighten when you do this. Keep turning the handle until the nut is tight enough and the fixture is stationary.