When you have found a template you wish to use as a basis for your trip in the trip template gallery, you just chose the menu item "Create new trip"
Which brings you to this page:
Every thing here works just as it doe's when you create a new trip, exept that all the information from the template owner is filed in exept for the trip time. But you can still add more waypoints in the waypoint menu, change description, invite friends etc.