Alright- i have written plans for dirt jumps, but my ramp and grind box pics aren't on the web yet. I'll try for them soon...
First thing you need is a place to build them and a run-up. If you don't have a run-up you can't jump them. To start off you want to place the dirt piles about 4 feet apart. To make sure of where you want the landing pile, you may want to build the first jump before the landing ramp. That way, you can jump the launch and find out where you land, then place the landing ramp there. To form the landing ramp you need to make it straight, not sloped like the launch. To make the launch, you need to slope the dirt, but not too steeply. Next, you need to moisten all the dirt to keep it well packed. (obviously you're packing the jumps). the jumps ahould be so packed they should look like dirt, but feel like concrete. You can use a flat-faced shovel, or a bike tire, even your foot. whatever you can do to pack it down. You may need to wait a week or so before you can ride them, and whatever you do, NEVER ride them wet- it grooves them up and tears them apart. Good luck, and happy dirt jumping!