I rarely approach anyone IRL, don't want to come off as creepy+rejection fears. Dating apps worked out for me but takes time to understand how they really work.
I have been approached IRL sometimes though, and such situations require a decent understanding of body language. It basically starts with eye contact. Someone who's interested and available will frequently look at you and try to make eye contact. That's a sign to try talking to them.
But the problem in such scenarios is there will likely also be some competition from other people around. And it's possible they lose interest once you actually approach.
Winning strategy is to become approachable so you don't have to actually make a lot of effort. Polish yourself, be it appearance, hygiene or conversation skills.
Conversation starters -
Easy (boring) mode: ask them their name, hometown, what they do etc. Compliment them.
Hard (exciting) mode: try to guess the person's attributes and make conversation focused on the present and not the past. Ask them random, funny and unpredictable questions. Humour works well.
Don't show too much interest, and be prepared to actually walk away too (in case it's a bad vibe). Some will prefer the easy mode approach vs the hard one because it's more familiar to them. Upto you to figure which one they like on the fly.
Tl;Dr - direct IRL approach is complicated and requires some prep.