Read the manufacturer's guide to your pressure-washer. There is a variety of models and each operates differently. Confirm that the motor is small enough to feed from a rain barrel.
Make sure that you have enough water for your job. A pressure-washer can only operate from a rain barrel until it draws down to two-thirds full. At this point, gravity is not sufficient to supply the water needed for the machine and the motor will begin to break down. Use the largest and fullest rain barrel available to ensure that you do not run low on water.
Connect a hose filter to the hose bib of the rain barrel. The particulates from asphalt shingle roofs make their way into the water of rain barrel. These granules are abrasive to the pump on your pressure-washer and you must filter them out. Purchase a filter at your local hardware store for less than $20.
Run a hose between the filter and the pressure washer. The hose must be sound. Old or stressed hoses can burst under pressure. Verify that all connections are tight so that water is not lost to leakage.
Start the pressure-washer and use a test spray to determine if water is flowing through the filter and hose correctly. Listen for any unusual high-pitched sounds that might indicate that the motor is starved.
Commence pressure-washing while keeping an eye on the water level in the rain barrel. With a 55-gallon rain barrel, this can occur in less than 10 minutes. If the level drops past two-thirds full, stop the pressure washer and reconnect to another water source.