Foundations can settle, concrete can expand or contract and faulty previous workmanship can create a host of problems with a basement floor. Uneven floors make it difficult to lay carpet, tile or other floor covering and they also affect the aesthetics of a finished basement. The main problem with a basement floor being uneven is that it's generally made from concrete, which is a hard, unyielding material. In order to level a floor that won't bend, you have to add material to the low spots which will balance out the high spots.
To begin, you must remove any covering from the concrete itself and any furniture, baseboards or other items on the floor. Remember, your floor might wind up being higher than when you started because the high point could have risen above the original plane of the basement.
The material you will use in order to raise the low spots up to be level with the higher spots is called Self-leveling Compound, or SLC. It's designed to be mixed with water and poured across the floor. It might require some raking with a gravel rake, but it's an extremely heavy compound that lets gravity pull it into a level position. It also dries very fast, so you have to work quickly when dealing with SLC. You also need a fair amount of skill to do this work properly, and it is a great idea to have help, especially professional help.
First, sweep the floor after clearing everything away in order to remove any dust and debris. It's also a good idea to use a high-powered, industrial wet-dry vacuum cleaner to get rid of all the dust.
After that, fill in any cracks in the concrete with concrete caulk, which you can do with any regular caulking gun. Remove any excess caulk with a trowel and pitch it. Next, build a dam by running expansion tape along the entire seam of the basement where the floor meets the wall. The tape should cover the wall to the level of the highest point, and it will control the flow of the SLC once it's poured.
Now, it's time for the heavy lifting, so to speak. Mix the SLC in a wheelbarrow. Add the water slowly in order to keep the mixture from becoming too wet. Stir constantly with a wooden paint stirrer. Remember, you must work quickly before the SLC dries. Starting at the far end, pour the SLC over the floor in strips about a foot wide. As you pour, have your teammates spread the compound into the corners with trowels. Spread the mixture around with the aforementioned gravel rake as you go.
Once the entire floor is covered, let it sit for three days before walking on it. Yes, the material dries very quickly and becomes impossible to work with, but that's only at the surface. It still has to dry underneath all the way.