If you're fortunate enough to own a large parcel of property and have been thinking about creating some singletrack trails throughout the acreage, it's a good idea to learn a few basics about building off-road trails. It's a lot of work, but it's worth it in the end. * Note - this post refers to singletrack trails, not a motocross track - here's a link to another post that looks at Building a Backyard Dirt Bike Track.
Soil: The type of soil makes a difference in your trail's design, due to the all-important aspect of drainage, so try to find out the soil type and decide what you'll need to do to keep the water off the trails. Sandy soil drains the fastest but doesn't hold its shape, loamy soil offers the best overall consistent drainage and traction, and clay soil holds its shape well but is really slippery when wet and doesn't drain well, if at all. The USDA has a soil survey page on their website that can help determine your soil type.
Leave the trees: Ideally you'll want to leave as many trees as possible - not only from an environmental and anti-trespasser (hiding the track) standpoint, but also because although going fast is fun, it gets really boring after awhile. Weaving in and out of trees is more challenging and will improve your riding skills. You can always cut the trees down later, but once they're gone, that's that. When clearing the trail, be sure to consider the trail from a dirt bike perspective, meaning check for low-hanging branches. It might be funny in the movies, but there is nothing funny about hitting a branch head-on in real life, even with a helmet on. Also look for short dead branch stubs (aka 'hat racks') and cut them flush with the tree's trunk.
If the property is heavily wooded and you're not familiar with the flora, fauna, and trees, either get a field guide or better yet, have an arborist or logger come out and walk the property with you. You may find a logger would be willing to cut down the trees for free (and may even pay you if the trees are in high demand). The big drawback to loggers is the mess they need to make in order to get their equipment in and out of the site. They're definitely not dainty!
Chainsaw basics: If you plan on doing the treework yourself but aren't very experienced with operating a chainsaw, do yourself (and your family) a BIG favor and read a few books and watch a few videos before starting work. It isn't only the chainsaw that can cause injury, it's also the falling trees. More info on chainsaw safety is covered in the Chainsaw Safety post.
Singletrack trail safety: It may be tempting to build a lot of jumps or switchbacks right from the start, but keep it simple in the beginning. You can always make it more challenging in the future once you and your friends have ridden the trails a few times and you've found you can handle the terrain. Depending on how extensive your trail system is going to be, marking the trails by degree of difficulty is a good idea. Consider some kind of color-coded system similar to the ski industry. Err on the side of caution, meaning if it would be considered an easy black diamond but a difficult blue square intermediate trail, mark it as a black diamond. You can always change it once you get some feedback on the trail's difficulty level.
Trespassers: Unfortunately, some people seem to think that what's yours is theirs, especially if the trails will be any distance from your house and they can access the property unnoticed. Try to incorporate the trail into the natural landscape as much as possible so the trail won't be easily visible (another reason not to cut down all the trees). Posting the property is a really good idea, and take pictures and videos of the steps you took to secure and protect the property so if anyone rips down the posted signs you'll be able to show they were in place. Just putting up a posted sign might not be good enough depending where you live, so check with your State's DNR governing body (or your attorney) to see if you need to put your name and address on the posted sign. If you see signs of trespassers and want to set up trail cams, make them as inconspicuous as possible because they'll either break or steal them.
Trail building books: Books are a great resource, but there aren't many books on the market about building dirt bike-specific trails. Mountain bike trails are somewhat similar is basic design, so if you can't find dirt bike trail books, try looking for mountain bike trail books.
Park Guidelines for Off-Highway Vehicles by George Fogg is more of a "big picture" book about planning an entire off road park, but it has some tips about designing and building trails. It was published in association with the National Off-Highway Vehicle Conservation Council. It's not really a mainstream book, meaning you might not find it on Amazon or at Barnes and Noble bookstores. It's fairly expensive (currently $46) and is available from the NOHVCC website.
IMBA's Guide to Building Sweet Singletrack is published by the International Mountain Bicycling Association, and covers pretty much all aspects of building mountain bike singletrack trails. The first couple of chapters deal with safety and paperwork, but the rest of the book is all about getting your hands dirty. As with the Park Guidelines book, it's not cheap, but there are usually used copies available. It's also occasionally available on Amazon or Ebay. Of the two books, the IMBA book is a much better choice for getting started with building singletrack trails.