Concrete sub-floors are notorious for having bumps and ridges, while wooden sub-floors can often be twisted out of shape by settling floor joists. Before you install any flooring, you should flatten imperfections with self levelling floor compound, especially if you are installing ceramic tiles, which can crack if there is a bump underneath. Self levelling compounds are easy to spread; they set quickly and are sand-able so you can get the sub-floor just right before proceeding with the flooring installation.
Clean the sub-floor with a vacuum cleaner, then roll on a coat of latex floor primer. Be sure to use primer specially made for this purpose. It is available from a flooring dealer. This will ensure that the levelling compound adheres to the sub-floor.
Use a straight edge to identify depressions in the floor and circle these with a pencil to mark them.
Mix the levelling compound with water in a 5-gallon bucket, using a drill with a mixing hook attachment to agitate the mixture. Add water, according to the specifications on the container, until the mixture has a soupy, pourable consistency. Work quickly, limiting the mixing time to two to three minutes because the mixture will begin to set immediately and you'll only have 10 or 20 minutes before it is too stiff to use.
Pour the compound into the areas you circled and let it stiffen slightly, then use a trowel to flatten it and feather it into the rest of the floor.
Immediately clean the bucket and spreading tool with water after mixing and applying each batch and before mixing the next one. Once the compound begins to set, you can't reverse the process by adding more water, and compound from a previous batch will contaminate a new one. Moreover, if the compound hardens in the bucket or on your tools, it is very difficult to remove.
Let the compound set for two to four hours before walking on it and for 12 to 24 hours before you begin installing tiles or other flooring.
Remove bumps or ridges in the levelling compound by sanding them down with a belt sander. Vacuum away all sanding dust before you begin installing flooring.
If the levelling compound becomes stiff in the bucket before you have used all of it, don't try to add more water. This will break it up and render it unusable. Instead, discard it, clean the bucket and make a fresh batch.
Don't lay tiles over levelling compound until you are sure it is completely dry. Tile adhesive won't adhere properly to wet compound.