Pour a little lacquer thinner in the cup, replace the lid, and shake.. Now pour that out, and wipe the inside of the cup with paper towels.
Pour another 1/4 of thinner, replace the lid.
Now turn the fan adjust knob to shot a tight pattern, and the volume knob to max, and spray the thinner through the gun. Occasionally blocking the spray nozzle to push air back into the gun. Caution, if you have a press on lid it has a tendency to pop off, make sure the vent hole is clear. I tend to do this without the lid on, just swirling the thinner in the cup. Do this a few times.
Now pour the thinner out, and put in some fresh. Repeat. Keep doing this until the thinner runs clear.
Take the sprayy nozzle off, and wipe it clean, using the pipe cleaner thing that should have come with the gun.
Then I tend to take the spray nozzle off, and put it inside the cup. pour in a about a cup of thinner, or just enough to cover the nozzle.