This is quite an easy tutorial, but also one that I've just figured out on my own. When creating a custom chain, it stored everything that the object has on it, springs, motors, hinges, other chains.

So, let me get started, first, you are going to want to make a square or a circle, any size.
Then add one hinge to each side.
After you have two hinges, add some springs. You can add how ever many you like.

After it's all done, it should look like this.


Now, select everything, and over on the left menu, it should say "Use As Chain", click it.

Now, draw your trampoline, you can make it wiggly, or straight by holding shift while dragging.

You can add anything else if you want.

Here is a video, resulting in the final product.
Phun Video