My understanding of the flank roles goes kinda like this: you want to be advancing or holding areas that the main combo (generally Reinhardt, a support class, and an offensive class, with maybe an occasional 4th) isn't actively looking at. This doesn't have to mean that you should be super far away from the combo either; you can have eyes on the flank while supplementing the combo's damage. You don't want to eat the same spam damage that the combo is getting hit with, but try not to be so far away that you can't swing over and lend a hand.The name of the game here isn't necessarily to have crazy high kill counts or damage, but to prevent the enemy flank players from coming at us from the sides or the back. Offensively, don't try to take on full health targets unless you're confident you can win the fight (you can get close and out DM them, they're distracted, etc), and generally going for picks on injured targets will be very effective (think about getting jumped on by a Winston when you only have 100 health left. You lose).
If you're playing a flank class and you hear that one of your main offense classes is going to push with an ult or that the combo is about to go crazy offensive into the other team, that's your automatic cue that you should be moving in on the enemy flank to distract OR to get picks on the players that are distracted by your combo. This is one of the things Frost and Sabey were trying to get at in the other thread: ideally it'll become an unspoken understanding that this is how effective pushes work. Combo in front of you? No problem, spam some damage at them with help from your flank. Flank pushing in? Focus them down, it'll only be one or two players. Combo pushing in with one or both flanks attacking as well? Better hope they can't aim.
Another super important flank player aspect is knowing when to RUN AWAY. If you're going to die, back out and let everyone know that you're no longer on that flank. This allows the combo to readjust their priorities while you dip out and get some healing and maybe your flank buddy can swoop in and save you. Learn health pack locations. Take enemy health packs so they can't have them. Get in their heads so much that they don't even want to be on the flank when you're there.
Finally, know that flank players are typically the most flexible. If you've been patrolling your section of the map and you know their flank players are dead or otherwise occupied, and you hear your combo asking for help, go help. You probably move very quickly and either provide a big distraction (Winston) or a lot of damage (McCree, Tracer, 76, Pharah, etc) that can quickly turn the tide of a combo vs combo fight. This doesn't mean give up the flank if you're in the middle of a fight; if someone asks for help and you can't come, say so. If you ask for help and nobody can give it, maybe it's time to fall back.
I might be missing some elements with this post since I'm new to Overwatch too, this is just general flank stuff I picked up playing TF2 that I'm trying to apply here. I'd also like to say that Frost is super duper cute. I love the haircut.