Trendline
A trendline connects at least two lows (uptrend) or two highs (downtrend) to show the slope of the market.
In an uptrend, you connect successive lows: the line acts as dynamic support. In a downtrend, you connect the highs: it acts as resistance. Two points are enough to draw it; a third touch confirms it.
The steeper a line, the more likely it is to break quickly. A line forced to fit, ignoring awkward wicks, loses its value: be consistent, either on wicks or on bodies.
A trendline break signals a slowdown, not necessarily a reversal. It becomes meaningful when it comes with a change in structure. Redraw your lines as new swing points form, rather than holding on to an old one.