Go to the Ocean Docks in Los Santos and look around. The Packer looks like a truck with a ramp on the back. Get any car and park it on the back. If you run out of room, use the Right-Analog stick and pull back. The ramp on which all of the cars you just positioned will rise, and you can park more cars. If you load cars on the end too far on the edge, they will fall off. Then, you can put it on the bottom.