Begin by filling your compound pan half full of mud (joint compound). Using your 6 inch joint knife held at a low angle to the wall, spread mud over the fastener rows such that the mud is compressed into the nailhead dimples.
Now scrape the extra compound from the surface of the drywall, with a stroke perpendicular to the direction you used to apply it. The knife blade should be held at about 45 degrees angle to the wall.
After this step is complete, you want to be left with nothing on the wallboard except the mud in the nail dimples, which should be flush with the rest of the wallboard surface.