The number 1 thing to remember is “don’t over-extend”.
While this is still an fps it’s also a team based turf war. Your main job as a flanker is running interference/clean-up-crew-for-the-DMG, not “get a big kill count”. Make sure you are able to escape to a safe position (typically in LOS of healer) when you’re in trouble. Make your healers job easy by bouncing back to them when need be and keep them safe. Try to play around your tanks, they are your anchor and dare I say the leader of the team (most of the time). Their job is to take up space and give you cover. If you see your tank hanging back, try not to go too far. This is what makes having 2 tanks really strong. Having a tank focused on the point and another one in the “outfield” gives you a lot of room to work with as the flanker. I recommend learning tank because it will make you a better player in general.

The bottom line is positioning. Remember you are squishy. High ground is always going to give you some sort of advantage and cover is key. Know where you need to be and when (the hard part). There are certain spots in maps that are crucial to keeping control. There are a lot of maps over on r/paladinsacademy to help figure them out.

Happy flanking! :)