So the system isn't explained all that well in-game, but here's what you do. When playing, press up on the D-pad, and you'll see a full  green bar in the lower right of the screen, slowly draining. Next to that you'll see a type of trick, like "drop" or "grind", with a number next to "next gap".  Basically, you have until the green bar runs out to do whatever it tells you, grind something for a grind, ollie down something for a drop, etc. Each time you successfully do this, the number will decrease until you hit 0.  After you've hit 0,  you can see the next gap on the map screen, scrolling between gaps using the D-Pad.

Lather, rinse, repeat.