Basically it ^^^
You eventually get a feel for everything after you paint a lot.
[one direction]
Flares are just nothing. I'd probably say, pretend you're goal is to paint 1 straight line [ starting from top to bottom] . After 2 inches of trying to get that straight line, Flick your wrist downwards. Now when people say "flick your wrist".. They don't mean Just go all wylin' out type shit. Just a cool "flick". What that basically does is it broadens the area & surface painted [Gradually of course]. Basically if you look at any flare, it'll start from one width and gradually just get bigger.
[curves]
Then there's the flares for when you do....U's or C's. basically anything you'll do curvey. You just basically do the same thing as stated for one direction'd flares, except after you flick your wrist away from the wall, (You'll only be getting half of the effect). So you'll want to bring that flick-of-the-wrist type of movement back to the wall (to get back to that standard width you started off with)
Practice one direction'd & curved flare .. going North, South, East, West.... then the diagonals..NorthEast, SouthEast, South West, North West