Thinset mortar is a type of cement used for the adherence of tiles and flooring to substrate, or subfloor materials. Tile installation requires grooves in the thinset to create air pockets which facilitate the adhesion of the tile. You can use thinset to install tile over an uneven cement floor and leave the floor perfectly level. You can also use thinset mortar to level out an uneven cement floor or fill small holes in the floor without installing tile.
Tape a string down from one high point in the floor to another high point in the floor. You may want to tape several strings down, criss-crossing the area so that you can see all the high and low points.
Pour thinset powder into a bucket and add water. Mix the combination until it has the consistency of peanut butter.
Spread the thinset over the surface, filling in the low spots and holes. Fill the spots until the thinset is level with the string. Smooth out the surface of the thinset with a finishing trowel, which has a smooth edge.
Allow the thinset to set until it is almost dry. Scratch the surface with a utility knife to rough up the surface a bit if you will be installing flooring above the thinset. You want it to look like there are shallow cat scratches over the whole surface.
If you have a small area that dips down a bit and will be installing tile over the whole area, you can just lay the thinset thicker in that area to raise the tile up to be level with the rest of the tile. You should use levelling compound if you will be attempting to level an area larger than two to three square foot.