Attach the garden hose to the end of the pressure washer. The garden hose screws onto the washer just like any other hose attachment.
Remove all items from the porch, including chairs, flowers and anything else that is either in the way of the walls or that you don't want to get wet. You also want to close all windows and doors near the porch
Turn on the water completely, then adjust the pressure washer to a psi of 1800.
Stand outside the porch when using the pressure washer. If you are inside the porch you are too close and may actually damage the walls.
Work from the top of the walls down to the ground. Run the stream of water along one siding plank, then move down to the next. Make sure you get in between the plants to remove not only old paint but dirt.
Turn the pressure washer up to a maximum of 2500 psi if all the paint chips are not removed and repeat the process. Anything higher than this may damage your siding.