Before installing flooring on your concrete floor, you have to make sure it is smooth and crack free. You will need a floor leveling or crack filling compound. There are several that are just mixed with water and cement based so they are very sticky. Read the instructions on the package for preparation. Most will require that the floor be clean and free of loose concrete. Take a scraper and remove any high spots. Clean the area with a sponge and water. Let the floor slightly wet, this will help the floor crack compound stick.

Mix a small amount of filler. They have a fast setting time, often 3 - 4 minutes. If you mix too much, it will set up before you can use it. Apply the material with a large broad knife. Try to get it as smooth as possible. A second coat might be required.

Get up any excess so when it dries, it will not stick up through the floor covering. Also, clean your tools as soon as possible. If the crack filler dries on the tools, you might have to throw them away.