After a couple of years of putting it off, I finally decided to build my very own bike trail in my yard. I found a few web sites on trail building, and got started. This instructible shows guidelines and techniques that helped me a lot when building my trail.
Remember-- Never build any sort of trail unless you have permission from the land owner. Even on public land, it is illegal to build. Many great areas have banned mountain bikers from riding on trails because of illegal trail building. Mountain bikers are being watched under a microscope because of this, so don't ruin it for everyone-- get permission first.
With that said, lets get started.
Step 1: Find a Location
This is one of the hardest parts, especially if you live in a city. I live in the mountains, and have two acres, so I didn't have to go anywhere. You also need to decide what you want in your trail, and who will be using the trail. I wanted to build a main loop that was fairly easy for the kids in my neighborhood to use. I then added a couple of other branches off the loop that are very technical and challenge me. When scouting, ask yourself questions such as: Technical rocks or smooth dirt? Steep hills or mellow ones? Once you find a good general location, bring paper and a pen and walk around. Take notes of sections that would be nice to have in your trail (drop-offs, meadows, etc) as well as sections that would not be so good (big cliffs, dense forests, swamps etc.) Also take note of any obstacles that need to be removed. I had to cut out a few bushes and some small trees for my trail to work. The trail itself will only be about a foot wide, but i would recommend four to six feet of clearing for your handlebars to fit through. The faster the trail, the wider the clearing needs to be.