DISCLAIMER: I AM NOT RESPONSIBLE FOR ANY INJURIES OR DAMAGES OCCURRED DURING THIS PROJECT. That means try to be safe on the ladder, and try to be safe riding on the trolley.
OK let's get started!
Rope long enough to stretch the length between the two trees you chosen
A plank for a seat (or an old swing set seat)
More rope for making a seat
Winch or truck to pull the rope tight.
All of the materials I use in this project are reused/recycled, except the winch. It is recommended that you use recycled materials, too, but you don't have to.
Step 1: Findin' some trees.
Step 2: Loop
(I found the rope on the beach. Free is always better)