There is a thread about it somewhere, and some YouTube videos, but basically:
1) Go to a Toybox
2) Place a wind fan (you have to get this out of the Toy Box first, best idea is to play through the intro stuff with a few characters to get 16 spins so you are sure to get it, keep refreshing the toy box until it shows up, and then keep picking until you get it (can take 1 try or up to 16)
3) Use a car vehicle (any will do, some say it's easier with some than others, I never really had a problem - and if you are playing a car figure, obviously you don't need another car)
4) Inch your vehicle up to the wind fan until you start to float above it due to the wind (this is the tricky part - going fast enough to get over the rim of the fan, but not so fast you end up going over the fan to the other side)
5) Once you are floating above the fan, use the analog stick on the controller to do a trick in each direction (up, down, left, right)
6) Prop the controller in some way so the analog stick is held in one direction (to keep repeating one trick over and over)
7) Walk away, come back an 30-60 minutes later
8) Just for good measure, do all four directional tricks again (there are theories that doing the four at the beginning and end helps up your trick bonus), and then accelerate over the fan and land on the ground.
9) When you land, you will get an incredible amount of XP. If you don't hit 15, just jump on and go again until you do. Each level you increase you get 1 spin, which adds up quickly at 15 per figure.
It sounds more complicated than it is, but once you get the hang of it, it is pretty simple. Since leveling a character doesn't really feel worth the time vs. reward, I did this on all my characters at the beginning just so I could play and not worry about collecting thousands of little sparks. It's quite liberating to just leave 'em behind.