There are two ponts to kept in mind in drawing trendlines:

1. It must touch at least 3 points
2. In a uptrend, draw the line along the lowest points in the trend without letting the line cross through prices.
In a downtrend , draw the line along the highest points in the trend without letting the line cross through prices.
It is not necessary that you will be able to draw a trendline in all timeframes in all market. If youy can draw them following above basic rules, then see how price responses to it.

Trendline drawn by joining by just 2 points are not much valid. More touches more valid it is.

Very good way to describe how to go about it as it is important to know how to draw trend lines. They can actually form the basis of analysis as you can see the price movements thanks to them.