Ok, there are many different techniques for using the fib ratios, the most basic and commonly used being the retracements and extensions.
The fibonacci retracement levels are the 23.6%, 38.2%, 50%, 61.8% and the 76.4% price levels.
The level at which price retraces to after a swing high to low or vice versa depends on the strength of the trend and the volatility of the pair being traded.
Generally you will look for the pair to reach at least the 61.8% fibo level before taking a position and possibly setting a stop just past the 76.4% fibo level although quite commonly the pair will retrace at one of the lower fib levels.
You should use the fibo levels in conjunction with other price action analysis such as support and resistance, candlestick patterns, chart patterns, etc.
I personally will take a position at the 61.8% price level if if coincides with one other form Price action analysis such as a trendline however if using one of the lower fib levels such as the 38.2% fib level i would look for a much stronger signal combining a few trigger points such as a support line that has now become resistance, a trendline and possibly a candlestick pattern or something along those lines.
Now the extension levels again are completely discretionary, there is no complete science as to where the pair will go after it has retraced from a particular price level however there are again some particular levels traders tend to look for, the most common being the 161.8%, 261.8% and the 423.6% price levels.
Now as with the retracement levels you should be looking for these extension levels to coincide with some other form of price action analysis as confirmation.
so to summarise:
All fibonacci retracements and extension levels are discretionary and you can never be sure exactly where price will begin to reverse although some levels are stronger than others.
Fibonacci levels are stronger when combined with other forms of analysis.
Hope this helped