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Make sure all the nails are below the surface. Make sure the seams are level. If they're not, belt sand off the high spots.
Fill the gaps that are larger than 1/8" with skim coat. You will want to work fast because this is a gypsum-based skim coat and will dry within five minutes.
Work in the skim coat and feather the edges. Go back over it with a large mudding (drywall) knife for a smooth finish. After it dries, knock down the high spots with a random orbital sander.
Cover the subfloor with rosin paper to create a moisture barrier between the subfloor and the finished floor and will smooth out any imperfections. Tack it down with a hammer stapler.