- 1). Use floor cleanser and a scrub brush to get the uneven floor completely clean. Pick up the cleanser with a wet mop. Let it dry for 48 hours.
- 2). Lay the 2-by-4 onto the floor surface, sitting on its narrow edge at one end. Move it slowly over the floor's surface looking for spaces under the board to indicate where the floor is low. Mark around those spots, using your pencil.
- 3). Mix self-leveling compound powder and water in a bucket, in the ratio called for by the instructions that came with the compound. Use your drill and mixing bit to mix the compound thoroughly. It should end up in a very thin, watery consistency.
- 4). Pour the self-leveling compound slowly over the areas that you marked with circles, letting it spread out at each area you pour it. Have your 2-by-4 handy to scrape it over the compound if there are any spots where it doesn't spread out completely flat.
- 5). Do the whole floor. Work quickly because the compound dries fast.
- 6). Let the compound set for 24 hours before laying top-flooring.
Source...