OK these materials are redially available to me because I rock climb but most other materials are redially available at home depot and REI (I am in the USA so it might be different for you if you live elsewhere)that being said I AM NOT RESPONSIBLE IF YOU HURT YOURSELF ADULT SUPERVISION IS REQUIRED AND BY ADULT I MEAN RESPONSIBLE ADULT ALSO PLEASE JUST USE COMMON SENSE so with that all out of the way here is what you need...
1. rock climbing harness
2. 2 100 foot lengths of rock climbing rope or steel cable
3. 1 come along AKA hand winch
4. a couple caribeners (3-4 max)
5. a ladder
6. a large pulley
Step 2: Locate your trees
now if you have a large backyard with many trees as i do this is a relatively easy task but below is the tree formation you are looking for basically a tree formation in a "T" completely onobstructed by bushes and other trees (also i apologize for the horrible bitmap image i am not too good at digital drawing)once you have located your trees move on to the next step.
Step 3: Setting up rope 1
this is where your responsible adult comes in now depending on the height of your zip-line set the ladder up on the base tree climb the ladder and firmly secure the rope to the tree with good knots then walk the rope to where it is in-between the other 2 trees