Carpeting a room can make the floor surface comfortable and inviting, unless the subfloor underneath is uneven. Seams, cracks, holes and low areas in the plywood underlayment or standard wood floor will transfer those shapes right through the carpet and will be discernible as you walk over it. Liquid floor leveler is a very thin cement that fills in those gaps and depressions, with its liquid form naturally finding its own level. This method works for uneven concrete subfloors as well, but make sure the packaging specifies it can be used on concrete.
Sweep and mop the wood floor, getting it completely clean. Use a well-wrung mop to avoid saturating the floor with liquid. Let it dry overnight.
Go over the floor with a hammer and metal scraper, hammering down any nail heads sticking up and scraping off any splinters or other obstructions. Re-sweep to remove any debris.
Fit your drill with its mixing attachment. Put 2 to 3 inches of floor leveler in the bottom of a bucket. Add water in the ratio called for on the packaging.
Set the mixer in the bucket and blend well. Add a little more powder or water as needed to make a thin, watery compound. Don't make more of it than you can use in 5 to 10 minutes.
Pour the liquid into the seams, holes and low areas in the wood floor, starting at one corner and working your way out. Use your concrete trowel to smooth out any high spots as you pour it, but the liquid should generally find its own level. Do the whole floor, mixing more leveler as needed. Allow the leveler to set for at least four hours.