Williams%R is same as other indicators once again except in its reading. Unlike other normal readings, it has typical negative readings on its scale. The indicator oscillates from 0 to -100. Same as stochastics, even Williams%R too has an overbought as well oversold regions for getting the stock movement at any point of time.
The recommended time period for this oscillator is 14 periods. It also had a centre/mid line containing a value -50 with it. Indicator line remaining above -50 for certain period of time indicates that markets are in bullish mood and the consistent closing periods below -50 indicates that the security is in the bearish phase.
Just like Stochastic and RSI, if the stock reaches -20 i.e. top of the indicator, it gives us an indication that the stock may fall with a view of overvalue. But touching the top of the indicator does not necessarily mean that. To confirm the trend reversal, let the line comes back and cut the -20 to travel towards -80.
If the stock moves to -80 it might take a pull back to the top of the oscillator again. Once again, not necessarily very soon it touches -80. Wait till it bounces back above -80 before taking up a bull trade.