How to build dirt jumps. After a bit of searching, I found some good tips to build dirt jump ramps. This is from wikihow.com

• You want to find a place where you have enough space to accelerate and stop after jumping, and even more room if you plan on making a circuit. You will typically need more space to accelerate then to stop, however, I have had cases where the landing wasn't enough.
• Start digging a hole with a very sharp rounded shovel. Don't throw the dirt anywhere, think of how you want the ramp. if you want to be cool and jump over a hole, take the dirt from the hole and put it where you want the ramp to be. Also, put some on the opposite side for a landing.
• If You want a table-top like ramp, dig two holes, spaced fairly far apart. Take the dirt and pile it up in the middle of the two. After enough dirt, you should have a nice mound a few feet or more high.
• The best way to make a lip is to compact the back of the ramp and much as you can. Lightly compact the front with your body weight.
• Take your bike tire and do a few roll-up's as I call them. At a slow speed, roll your front tire up the ramp to make the lip. This also compacts the ramp more.
• The landing ramp should always be smaller than the take-off ramp. You don't need to do much for it, besides making it not as steep as the take-off ramp. Make sure it is compacted, otherwise your landing will hurt.
• To make the ramp hard, get it wet, muddy wet, but not enough to ruin the shape. Let the dirt harden in the sun for a few days, and repeat.
• Try the ramp out, and make changes. You may find yourself scraping some dirt off to make the lip more smooth.

Tips

• Getting the ground soft before you dig makes life a whole lot easier.
• If you have a small skid-loader, you can make ramps in much less time.
• Make sure it is hard-packed enough to support the weight of the bike AND you!
• To save on digging, you can make the core of the ramp with construction surplus, like broken concrete and bricks. Just cover them with a thick layer of dirt and pack it.