I use them as guides to resistance and support on a trend. While not perfect, they are useful. On the downtrend measuring from high point to low point on a trend it shows potential areas of resistance which can help in figuring out a target price on a swing. Drawing from low point to high point on an uptrend they show areas of support which I use to get a posture on how low a stock could drop. They are most accurate on indexes and ETF's but are helpful on individual stocks as well. I use Fibonacci every day on the S&P
If tracing a down trend, start at the top and pull to the low point and on an uptrend start at the low point and pull up to the high point. I should also mention that what you look for is a bounce of each of the lines you see. It the trend passes through a line it is likely to go to the next line or is at least free to do so. The bounce off the line helps confirm it is acting as support or resistence.