You need a strong tree with at least a 30cm diameter that is higher or level with the ending Tree
You will need a height of at least 2 meters. From the floor.
You need to make sure you have a direct line of sight to the ending tree.
The drop in level from the starting tree to the ending tree will depend will on the length of your zipline. But at least 1 meter for every 20 meters.
Your kids need to be able to reach the zipline handle 30cm below the zipline, so you may need a platform of sorts for them to start from. A couple of wooden logs will do or you can build a platform of sorts to help them launch.
You will need to decide on the length of the Tree Protector rope for the starting tree. We have even wrapped this rope around 2 or 3 branches to get the zipline in the perfect line from start to finish.

The ending tree also needs to be at least 30cm in diameter
The height of the end of the zipline is what you will use to slow down the zipline or speed it up.
Your kids may need a platform of sorts for them to get off the zipline. A couple of wooden logs will do or you can build a platform of sorts for them to climb off. 

The Breaking system is key to making sure no one gets hurt..

We have a Spring with a soft velt washer for low noise on impact and a rubber washer on the tree end to make sure the coil does not creep along the rope around the tree.s.
Before you secure the zipline rope on the end tree make sure you have fed the rope though the brake coil. Velt washer 1st, then metal washer then coil, then rubber washer.
 

Some ideas of a launch pad, i built a little 4 poll platform but you can really get creative and use what best works in your garden.