Circular or handsaw
Five-foot piece of string
At least six eight-foot lengths of 2'x 4'
3 pieces of 6' x 4' sections of plywood
First, you need to sketch out the slope of your half-pipe. To do this, you need some simple geometry, a piece of string, a pushpin, and a pen. To hold the string in place, pushpin one end of it into the ground or a piece of cardboard. On the other end, attach a pen and pull the string taunt. Starting at one corner of the plywood, you will draw a line--or, more accurately, an arc--along the plywood. Keeping the string taunt at all times, swing the pen in an arch and sketch out the slope of your ramp directly onto a piece of plywood.
You really only need to swing the rope about 45 to 60 degrees from its starting point to make a comfortable arc--any more and the slope of your half-pipe will be way too steep. dude.
Using the jigsaw, cut along the traced line. Cut out two identical pieces of plywood. These are the sides to the ramp. From here on, it's pretty much bang-bang-bang. Cut four-foot lengths from the 2x4s. 12 should be enough. Nail one length on each corner of your plywood. Then along the bottom and the back edge bang in 2x4 crossbeam supports every 18 inches or so. On the ramp, nail a 2x4 every eight inches to a foot. Make certain that the 2x4s are flush with the lip of the plywood. This will give better support and--voila!--a smoother ride. This is your completed frame.
Take a piece of plywood and gently arch it along your ramp; nail down the edges. If you moisten the wood, it will bend and curve easier. (Don't take bone-dry, sun-bleached wood; it will crack, and those are splinters that will wind up in your ass.)
That's your half-pipe. Use the remaining scraps of wood and plywood to build a table to push up next to the back side of the ramp, and you'll have a complete half-pipe that you can ride over the lip and onto the tabletop (Wow! Just like Grandma used to do!).