What is the best way to install track outdoors?
There are dozens of different methods suited to different people and different situations. You can even just lay your track on the grass and run your train right on the lawn. For permanent installations, however, use the "floating" method. The track floats in a bed of gravel, called "ballast," just like real railroad track. Pour a roadbed of coarse gravel where you want to put your track. Then set the track into the ballast, level it and pour a bit more ballast on top. That's it. (If you want to get fancy, you can dig a shallow trench to hold the ballast in place.) Rainwater and snow melt will drain away from the track, and the track can move when it contracts in the winter and expands in the summer. Plus, it's easy to change your layout when you use this method.

What kind of gravel should I use for ballast?
Different materials are available in different parts of the country. Look for gravel about 1/4 inch in diameter. In many areas of the U.S., farm supply stores sell gravel in this size as "turkey grit" or "chicken grit." (In Germany, look for gravel, called "vierer Körnung," about 4 mm in diameter.