Decide what constitutes an emergency. You'll probably want to activate your phone tree for circumstances such as cancellation of a scheduled event or a change in the time or location of an event.
Determine who needs to know the information your phone tree will communicate.
Make a list of those names and collect contact information. Get two phone numbers (home, office or cellular phone); at least one should have an answering machine or voice mail function.
Choose who will sit on top of the tree (the team coach? the band director?). Work with that person to decide how many "branches" you need.
Recruit volunteers to work the branches, and assign each of them a specific group of "leaves." Be sure your branches have all the necessary contact information for their leaves.
Test your tree at least once before you need it, and fix any broken connections.