Map out the design you want to create. Draw out all the specific loops and turns and determine where supports must be positioned.
Mold the roller coaster using grey pipe insulation for the track and cardboard tubes for the supports.
Cut open the insulation to make a track where you can watch the action.
Tape the supports onto the insulation using duct tape.
Perform a trial run with your track using a marble. If the marble makes it through the roller coaster, then your model worked. If not, modify your roller coaster as necessary.