its fairly simple.
Swing low/high on the chart recent data only.
by that I mean the current swing cycle.
Understanding of the market structure is important. so when you look at it you can first identify the trend then look weather market creating (lower low) or (high highs) after knowing the structure you can draw Fibb and then wait for the retracement accordingly.
Just pick up the fibb and start trying it on the trading view or mt5 until it starts making sense for you.
Watch some youtube videos as well.
I hope that helps.