well, when you want to start building a dirt jump, you will want to find a nice piece of land, a spade shovel, a wheelbarrel (or a 5 gallon bucket), a few friends, and a saw, or brush clips. Start by clearing away any small trees (anything smaller then 4 cm in diameter) that is in your way. After all that is cleared away, find some nice top soil (not sandy soil if you can help it), and start by making a pile about 3-4 feet high, packing it in every once and a while.
After you got a nice pile, start shaping it in a arch somwhat like this:
Now, get some more dirt, and start making your landing ramp. it will basicly be the opposite of the take-off lip, but dont build it as steep as the back wheel may catch on it, making you go over your handlebars. build it about 6 feet in front of the take-off jump that you have built.
Now get a bucket of water (if possible) and slowly pour the water onto the ramps. This is for packing the jumps in, making it twice as hard as it would be. If you don't have water avalable, you will have to wait until it rains.
Now, I know you can't wait till it rains, so, really try to pack it in, stomp on it, jump on it, hit it with your shovel, and get it packed in!. When it is all finally packed in, I bet you will want to try it out. Go fairly fast when you hit it, enough speed to clear the jump, but not too much speed to overshoot it. Try to push the front wheel into the landing ramp (so you come down on both wheels at the same time). Now, I know that you will do the exact opposite of that, you will overshoot the jump 2-3 feet, and you will come down on the back wheel first.