Applying an aerosol belt dressing is as easy as it seems. All you need to do is aim and spray. The catch is that you have to do it with the engine running, so be very careful!
You need to direct the spray toward the inside of the belts, the part that touches all of the metal pulleys. Since the belt is moving, you only need to find one good location to spray from. Spray the entire length of the belt by holding the nozzle down for 10 seconds or so while the belt goes by.
- Work on level ground with good footing so you don't lose your balance.
- Don't wear hanging garments that could get caught in a belt and suck you in with them.
- If you have long hair, don't lean over the belts close enough to get your hair caught.
- When working in a running engine bay, always concentrate on the job and avoid distractions. If there is too much going on around you, wait until you can do it calmly.
Remember, this is a temporary fix. Your belts are squealing because they're worn or loose, and should be properly repaired asap.