Here is a possible idea for a jump. This isn't meant to be an exact blueprint but more of a guide so that you can make the jump so it suits you best. You can make it bigger or smaller, whichever you prefer. We made it this size because it's as big as we could make it while still being able to fit in my truck.

This picture shows the skeleton of the jump. The black and gray lines represent 2X4's. I recommend using 2X4's so that your jump is nice and sturdy. It's easier to nail the boards indicated by the gray lines first.

Nail the 2X4's like so. It's a little tricky to find the center of the 2X4 when hammering down through the other board though.

Once again, nail these 2X4's first because the 2X4's colored black in the picture just support the plywood that will go on top and aren't as structurally important as the gray ones.

Get a piece of 1/4 in. or so plywood and soak it for hours in water. It then should be somewhat flexible now. Get a couple guys to stand on it while you nail it on to the black 2X4's.