Accumulate a good sample size of hands and look for statistics that are very different from optimal.
Common high impact leaks to begin with are:
- Opening wide/tight (review VPIP)
- Calling too much preflop (review VPIP/PFR ratio)
- Being passive postflop (review AF)
- 3-betting too tight (too loose is very uncommon but might also be a leak) (review 3-bet%)
- c-betting a lot or not enough (review flop c-bet%)
- calling c-bets too much or not floating enough (review flop fold vs c-bet)
If you're not sure about any of those spots start by studying the specific topic.
As your understanding of poker increases, you naturally find other spots where you might be playing a suboptimal strategy.
During a session, I always mark the hands where I was unsure about my strategy. Reviewing those specific hands will optimise your strategy for similar future situations.