Step 1: Design
Unless you're making a circle shape, it's best to use an octagon shape while following a square scaffold.
When making the octagon shape, try making all edges the same size. It's not essential, but it helps later.
Also decide how wide you want it.
Choose some elements which can be used from any angle, such as the path drop, a Ferris wheel, or an arm drop.
Also make sure you consider that the ball machine will constantly rotate faster or slower than you want it to.