Most basements are made of concrete or masonry block. In many cases the masonry (or concrete) is completely dry. The cement wall should be clean and completely free of oil, oil paint, dirt or any other nonsense that will keep the paint from sticking to the masonry.
Lightly dampen the masonry surface with water before you start the brush-work. The idea is to keep the wall from drawing moisture from the paint.
The easiest technique here is to turn a garden pressure sprayer on the wall using a fine fog spray nozzle so that you moisten the area but don't soak it.
Mix the paint with water following the directions on the container. Cement paints are quite heavy and the ingredients tend to settle out, so keep stirring the paint as you use it.
The brush used for applying this paint is a stiff fiber scrub brush. This isn't a lightweight coating that you just flow onto the masonry surface.
Because the cement or masonry brick has many pores, you'll actually have to scrub the paint into the wall to get a decent looking job.
Dunk the brush into the paint and scrub back and forth over the masonry. Then scrub round and round until the area is covered. Plan to work on a relatively small section at a time.
If this sounds like a great deal of work, keep in mind that you're applying a permanent coating to your masonry. As a matter of fact, when properly scrubbed onto the wall and carefully cured, cement paint becomes an actual part of the concrete masonry surface.
There is one big pitfall to this entire routine of finishing walls. As noted, this phase of the job makes a big fat difference in the looks of the remodeled area. Because of that, you may have a tendency to rush the work; even skimp here and there, as you work on the masonry.