Make sure the area to be sprayed isn't near any electrical outlets or wires, and doesn't contain any glass or delicate fixtures that might be damaged by the high pressure spray.
Set up your spray washer according to its instructions, hooking up your garden hose and affixing the 15-degree nozzle.
Hold the nozzle about a foot from the area to be sprayed, at a slight forward angle. Pull the trigger on the sprayer to start the stream.
Move the nozzle closer to the surface, just until you see the paint starting to come off. Use slow, steady forward strokes to remove the paint, pushing the stream forward as if you were pushing a solid scraper. If the surface is wood, stay with the grain of the wood. Move the nozzle back if you see any sign that the surface is being gouged.
After the entire surface is stripped, allow several days of drying in the sun before repainting.