1st. Make sure that you are on a versus sesion by yourself.
2nd. Go to the area where there is a car and a van and some tyres to the side of the van
(the starting point for alpha team)
3rd. Line yourself up against the end of the fence next to the carand then look at the tyre closest to you.
4th. Run (LB) at the tyre and push yourself against the car and (probably after about 3 tries) then you will be on top of the cars hood.

other things about that

When you are on top of the car there is a van. If you want to get on top of the van then, simpily crouch (LS-click) and move slowly-{while pushing against it}-around the vans hood and then you will be able to walk up the front window, not just that, you can crouch again and then (at the back of the van) walk up to the tyres and you can go on top of them.(a brrrilliant place for hiding) oh and you can walk off the hood and onto the other side of the cars, there will be a white building further down the road to your left with a power line pole in the corner of it, the thing about this building is that you can walk into it (but theres nothing there) and hide there (and you can see through the wall, another brrrrilliant hiding place.

If having troubles doing this glich, please leave a comment and I could help you.

Happy gliching