First thing you need is the frame diameter. All you need to do is a grab a friend, and measure the outside edge of the frame directly across to the other outside edge of the frame. It is key to make sure there is no slack in the measuring tape when doing this, which is why you need someone to help you. Click on the links to see a diagram of how to measure the different shapes of trampolines (round trampoline , octagon trampoline, and rectangle trampoline).
Next you need to know how many trampoline springs are on your trampoline to. This is pretty easy to find, you simply need to count how many springs there are on your trampoline. It might help to mark the spot where you started with something so you do not get a wrong count. If you have a square or octagon trampoline, you may also need to know how many springs are on each side.
Lastly, you will need to know the length of the springs. You will need to take a spring off of the trampoline (a spring puller may help) and measure hook end to hook end. Try to find one that is the least stretched for a more accurate reading.
With this information you will be able to accurately choose the trampoline mat that works for your trampoline.