If you go to most established trails, the jumps are so big, you end up just looking up (and I mean, up) at them in awe with your jaw wide open - then deciding you'd rather just go get a Slurpee. Finding a 'rookie' trail is not easy, so like many riders, the only way you're gonna learn is if you build your own, and start small - real small.
Obviously you need a place to build them, that's relatively flat and big, has pretty good dirt (minimal rocks), and hidden from prying eyes. The next step is to buy the implements you'll need. The most important items of course are the shovels. You'll need several spade type shovels (those are the kind with a pointy tip), for digging and moving large amounts of dirt, and you'll also need at least one flat edged shovel for shaping the jumps. Cheap shovels can be bought at discount hardware stores for as low as $15, though sometimes you may want to spring for the next not-so-cheap-but-still-cheap shovels that go for about $25, they'll last just a bit longer. Depending on the spot you pick, you may have to move dirt from one spot to another. You could go the cheap route, and scavange for some of those big white 5-gallon plastic buckets, which can hold a pretty good amount of dirt, you'll just have to find some friends with good muscles. The best thing though, is to spring for the cheapest wheelbarrow you can find, which can be found for about $60. Then, depending on your location and time of year, you may want to buy at least a few cans of bug spray. Don't laugh, being eaten alive by hungry ravenous mosquitos can easily change your mind about building a trail.
Once you've got a good spot to build, shovels, and a few good friends to help, the next step is a plan, and the idea is to plan for the future. Don't build a big doublejump in the middle of your spot, because you'll find yourself running out of space in no time, and end up building new jumps backwards (in front of your first jump). Determine what direction you want to go, and where you're gonna start From there, you'll want a long enough straightaway to be able to pedal fast enough to make the first jump without any problem. Keep in mind that if you intend to get serious about dirtjumping, your jumps will get bigger and bigger each year, so you may need more space in the initial straightaway to get more speed for the bigger jumps. Also note where the trees are (assuming you have trees), and plan to snake your jumps through the trees. Don't start cutting trees down, for several reasons. First, it's really cool to jump through the trees, the trees give you shade, they keep the soil intact because of their root systems, and you don't want to cut trees down anyway, unless you're a retard and have some hatred for nature. Trees are also very helpful as a structural base to support a jump. Note that if you're building through trees, you can't have a pit (the area where you land from one jump to the takeoff of the next jump) next to a tree, because of the roots of the tree, so the idea is to build the takeoff lips or the landing lips next to the tree itself, with the pits in between the next set of trees.
Once you have a general plan for what direction your jumps will go, you'll need to measure distances so you'll know where to start building them. For beginners, the best size doublejump is no more than 2 feet high. This may sound wimpy, but don't kid yourself, you can break your neck on a 2 foot jump just as easily as on some huge one, and the skills required to jump a 2 footer is the same skills required to jump bigger jumps. Actually, to jump smaller, tighter, more technical jumps, is almost more difficult, for it requires greater precision, hence you'll learn your jumping skills better. The next measurement is the distance between the lip of the takeoff jump, and the lip of the landing jump, which for a 2 foot jump, should be about 8 feet (you don't need a measuring tape, your feet will work just fine). Now mark where you want the takeoff lip, the landing lip, and start piling up dirt.
How to build your first dirt jumps
Don't even try to shape the jumps yet, your only concern at first is to pile up as much dirt as possible. Even though the jump will only be 2 feet high, you'll be packing down the dirt, so you'll need a LOT more dirt than 2 feet worth. Make the jump about 4 feet wide, and use the best dirt you can find. Try to keep leaves and twigs out of the dirt, for if you don't the leaves will prevent the dirt from being packed down hard, and will eventually decompose and your jump will fall apart. Obviously you don't want to have rocks in your dirt either, cause, well, rocks hurt! Even small rocks, which may seem to blend into the dirt when you first build them, will get partially exposed after a few rains, which will be cemented into the jump like a giant cheesegrater, ready to rip your flesh away like so much Cheddar should you fall on it. Once you have some really big wide piles of dirt where you want them, start packing them down by walking on them. Still don't concern yourself with shaping them yet, your main concern now is to pack the piles down nice and hard, starting on the top, creating a flat top, and then packing the sides down (smacking the sides of the jumps with the back of your shovel helps too). Now that you've created two big packed down piles, your #1 double jump is starting to take shape. The next step is to shape the takeoff lip, which is the most critical part of a jump.
Shape your take off lip
The shape of face of the takeoff jump, is the same as the transition of a ramp. Nice and curved, round, with no flat spots. Take your flat edged shovel, and stand directly in front of the jump, and without moving your position, start carving the face of the jump from the top to the bottom, in nice smooth curved arcs. Be consistant, regular, and create a nice curve. Create the curve, or transition, so that it almost goes to vert. In other words, standing from the side of the jump, the top of the lip of the jump should be just short of pointing straight up. It may seem that with the lip so steep, you won't be able to clear the distance, but a little basic physics will prove you wrong, which is just another reason why you should be serious about school, for your physics, geometry, and math classes all come into play. If you roll a ball at speed at your jump, even though the lip almost goes to vertical, you'll see that the forward motion of the ball will send the ball arcing quite a distance, which is why you have the landing jump about 8 feet away. This is why it's so important to measure your distances before you start building your jumps. As you're shaping the jump, carefully pack it down by either smacking it with the back of your shovel, or having a friend hold you from tipping over, and pack it with your feet, being careful not to screw up the transition with your feet. When it gets pretty much packed down and the shape looks good, you can also use your bikes front wheel while sitting on the bike, by rolling it up and down the jump like a big breaddough roller, just be careful not to flatten the very top lip of the jump - keep the transition continuous up to the very lip. Once it looks pretty good, then repack the top of the jump flat, which will lower the jump just a bit, but then you'll have a good strong dense jump
Shape your landing
The next thing to do is to shape the landing 'jump'. This is a lot easier to do, for though you do want a transition (curved shape), you don't want it quite as steep, otherwise it'll be too tight, and you'll hang up your rear wheel too easily. Make the landing transition milder, and also make it a bit longer, almost twice as long. The landing 'jump' should be wide, and well packed down on the edges, so that when a rider goes a bit off course, and 'case' the landing, it won't get totally destroyed.
Now the only other thing left to do, is to water down the jump, that is if you have water available. What makes a jump rock hard is water, allowing the jump to be soaked, and then dried, essentially cements the jump. If you don't have water available, you'll just have to wait for some rain. Just remember, don't ride your jumps when they're wet, otherwise you'll groove the jump badly with your tires. Of course, you know after all that work you've done to create your first double-jump, you're not about to wait for rain, you're gonna test it out. Time for the fun part...
Dirt Jumping basics
Before you attempt to jump your first dirtjump, you need to understand the physics involved. If you look at your now shaped double-jump from the side, you'll see that the line of trajectory the object (you on your bike) has to travel, is a nice arc. You start going straight, then hit the transition on the takeoff jump, and arc smoothly and curved, hitting the apex at the center between the takeoff lip and the landing lip, then start pointing back down, to match the landing perfectly, just as if the double jump was filled in with dirt completely like a big round mound of dirt. There are two things you must focus on to jump correctly, 1) do NOT overjump, and 2) make sure you nosedive into the landing, so that your front wheel almost nosebonk's the lip of the landing. The key to jumping a double-jump, is to jump no higher and no farther than you need to just barely make the distance. If you overjump in a rhythm trail (a rhythm trail is 2 or more doublejumps linked together), you'll never make the next jump because you won't have enough room to pump. The idea is to jump very low, in what is referred to in BMX racing as 'speed jumping'. In BMX racing, the fastest way around a track is on the ground, not in the air, because pedalling in the air doesn't work too well, so the idea is to get back down on the ground from a jump as quickly as possible. On a dirtjump trail, the idea is to land as close to the landing lip as possible, so that you can immediatly pump into the landing pit and jump the next doublejump. Unlike in BMX racing, the only pedalling you'll ever be doing is at the start before the first jump, so it's even more critical that you jump low, land pointing nose down at the very lip of the landing, and immediatly pump to the next jump, etc., etc.