You make your own stage, heres the proccess.
Choose a size (out of 3) of how big thee level will be.
Choose a background (airbourne, medieval, futuristic)
Choose music (so many options)
Start building! Their are 3 categories of items: Flooring, Structure, Specials.
Floors have slants, blocks, stairs, and... floors
Structures have a bunch of cool looking decorations to paste
Features have Treadmills, Spikes, Pinwheels, Moving platforms, Ice blocks, Fallign blocks, and ladders.
Their are also part sets you can unlock (3 of them) that add 1 structure and 1 feature to your list.
You have a budget like on Halo 3's forge though, but it goes by space you taken up rather then money.
And their you have it!