Not all backyards are created equal. One man's weed-infested postage stamp is another's three acres of manicured sod. The amount of space you have available will impose one of the most significant limitations -- next to money, of course -- on the size and scope of your backyard skate park dream.
This would be a great time to include your spouse in the planning process. Not to generalize, but backyard skate park projects are often initiated by men, and men don't always consider the full implications of erecting a 12-foot by 16-foot (3.66- meter by 4.88-meter) halfpipe in between the rose garden and the detached garage. Your spouse may be able to provide some useful, if sobering, advice about how much backyard space can reasonably be occupied by ramps and rails.
Flat and level are two important words to remember. The cost and complexity of building a backyard skate park increase in proportion to the slope and unevenness of the foundation. If you start with a nice flat surface, it's a lot easier than trucking in gravel and fill and cement footings to level the property. Unless your backyard consists of a large, empty, bowl-shaped concrete swimming pool, in which case your work is done.
Next step, figuring out how much money you can spend on this skate park fantasy.