Find a good spot to build the half pipe. The ideal site for a half pipe is on level ground, well-lit and dry with few trees. Once the ramp is built, it's impossible to move so your construction will be the permanent site for the ramp. The half pipe is essentially divided into three parts, the flat bottom (the flat middle section) and the two curved sides. Start by constructing the flat bottom of the half pipe. Set your pieces of concrete at the four corners of the flat bottom and make sure they are level. Cut four pieces of 2x6s to create a square frame following your plans. Cut 2x6s and place them every eight inches on center to support the frame.
Draw the sides using a piece of string anchored to a screw as a compass. The curve on the sides is called the transitions and should be a continuous circle from the flat to the top of the ramp. This is true whether you're constructing a six-foot half pipe or a smaller one. Cut the sides and attach them to the flat bottom. Next, frame the sides with 2x6s, eight inches on center following the curve up to the top. Cut out the small notch at the top for the coping--the metal pipe that lines the seam where the transition and platform meet. Frame out boxes for the platforms on each side, carefully following your plans and making sure they are level and square.
Add the surfaces to the ramp and the platforms. Cut sheets of plywood to surface the flat and the curved transition. Use wood screws to anchor the plywood sheets to the 2x6 framing. Add another layer of plywood to the flat bottom and transitions, making sure to lay them in the opposite direction as the previous layers. Avoid lining up seams from the two layers. Cut off the plywood at the notch for the coping. Some plans call for a layer of tar paper before adding the final layer of waterproof Skatelite on the bottom and the flat. Follow your plans to finish by surfacing the bottom and transitions with a layer of Skatelite. Now you're ready to surface the tops of the platforms. Some plans call for the platforms to be surfaced with 2x4s, others with plywood. Using 2x4s will make the surface much stronger and more durable. Adding railings to the back and sides of the platform make them a safer place to wait and watch when not skateboarding.
Finally, cut the metal pipe to fit the width of your half pipe and place it in the notch cut out where the platform and the transition meet. Use a metal drill bit to punch four holes in the pipe. Use screws to anchor the pipe to the 2x6 framing the notch. Use a broom to clear any debris that may have accumulated on the half pipe and it is now ready to skate.