As you spawn or start the game, head towards one of the white trucks, anyone of them.
Now, get your pistol out (down on directional pad) and take cover on the back of the truck. Keep on walking/running left to right and eventually you will be floating in mid air, taking cover on the back of the truck.
Now, carefully, walk while still in cover and gently tip your left thumb stick to the right or left so you're in the middle of the cover.
Hold up on your left thumbstick and press A, when you're person is halfway up press it again.
You should Be floating on the cars if done right.
You may not get it the 1st couple tries... but it takes some practice.