Using an airless sprayer effectively takes some getting used to, so plan to practice on some scrap plywood or an inconspicuous part of the room or the house. The object is to cover the surface with a uniform coating of paint. Hold the spray gun a constant 6 to 12 inches from the surface and maintain this distance with each pass of the gun. Keep the gun precisely parallel to the wall. Don't sweep it back and forth or you'll end up with a wide arc of paint on the wall; the paint will be concentrated in the middle of the arc and almost transparent at each end.
Paint about a 3-foot horizontal strip at one time, then release the trigger and drop down to paint another strip of the same length, overlapping the first strip by one-third to one-half. Once you've covered a 3-foot-wide area from the top of the wall to the bottom, go back to the top and start another 3-foot section adjacent to the first, overlapping the edge of the first painted area by several inches as you work your way down the wall again.
Examine the painted areas to make sure the entire surface is receiving a uniform coat of paint. Too much will run or drip; too little will let the old paint show through. If you notice these flaws, it means you are not keeping the spray gun a uniform distance from the wall at all times or that you are tilting it. An upward tilt will deliver excess paint to the bottom of the painted strip. A downward tilt will concentrate paint at the top of the strip.
To prevent paint buildup at the end of each strip, release the trigger on the gun a fraction of a second before the spray gun stops moving at the end of your stroke. When beginning a new strip, start moving the gun a fraction of a second before compressing the trigger. Always keep the gun moving when it's spraying.
Be sure, too, that you've properly thinned the paint and adjusted the pressure control according to the manufacturer's instructions. If there is too much or too little thinner or too high or too low a pressure, the spray of paint won't atomize properly.