Drawing the gap
In a bullish FVG, the gap runs from the high of the first candle to the low of the third. If those two do not overlap, the middle candle left an imbalance. A bearish FVG is the mirror: from the low of the first candle to the high of the third.
Which gaps matter
Gaps are everywhere on lower timeframes. The ones worth attention formed during the move that broke structure, or right after a liquidity grab. A gap on its own is a location, not a reason to trade.