Sweep the entire floor. Then vacuum as well. You do not want any dust or debris left on your sub-floor; this can interfere with the ability of your self-leveling floor to form a good bond.
Apply a mild concrete etching solution if your sub-floor is concrete. If your sub-floor is not concrete, skip to Step 4. Carefully follow the directions on the product you have chosen. Acid left on a floor for too long can weaken the entire concrete floor and cause problems with cracking and adhesion later.
Spray the concrete floor with water from a garden hose to clean the acid off. Do not use a pressure washer. The force of water from a pressure washer will drive debris down into the concrete, and this will cause problems later. Allow ample time for the concrete floor to completely dry and the added moisture from the washing to dissipate in the air.
Mix your self-leveling floor compound in a bucket. Carefully follow the instructions on the product you have chosen. Some compounds are mixed with water only; others require special liquid additives to prepare them. The information will be on the packaging of the compound.
Pour the self-leveling compound onto the floor and use a squeegee to spread it over your sub-floor. A good rule of thumb to remember is that you only want about an 1/8" of the compound on the floor unless there is a dip or rise in the subfloor. For both of these instances, leave about a 1/4" of compound on the floor. As it levels, it will smooth into the flat areas.