Select Time Trial from the Main Menu, Moto GP, and then any rider you so desire. Choose basically any track with a long enough front stretch (I chose Mugello, I was going for the Wind Tunnel achievement at the time), and launch the game.
From one end or the other of the straight at a stand still, accelerate up 150 mph or so, and then pull back on the left stick to initiate a wheelie. Just keep holding back on the stick until your rider goes over the back of the bike, and the achievement should pop after a few seconds when the game has put you back on the bike. If not, you'll need to go faster before falling off, or you need a longer straight to do so on (as I said, Mugello works perfectly).