Trend lines are the most common form of technical analysis used today.
If drawn correctly, they can be as accurate as any other method...
An uptrend line is drawn along the bottom of easily identifiable support areas. In a downtrend, the trend line is drawn along the top of easily identifiable resistance areas.
I have attached a chart with an example.
For more details on T.A refer to thread teach a man to fish.... as said by satya pinku.
Regards,
Luckytrader.