We purchased a set of plans from Kuffel Creek to mainly get the process in one place. If someone invented the wheel, might as well use their ideas rather than having to invent our own.
One suggestion though, cut out the profile of the trailer you want to build and put together a working prototype so you can see what you're making!
This is ours:
As you can see, we're not using the Kuffel Cubbie profile. We ended up melding two profiles together (thanks to Photoshop) and projecting this one on to the cardboard, cutting it out and putting it together. Ours is a Kuffel Cubbie front (up to about the front door) and, uhhhhh, something else. Then we took the entire image and pushed it around in Photoshop. This process gives you a chance to see in real scale how big (or not) everything is and where it goes. Best of all, it's not as expensive as doing the whole thing in plywood to realize that you don't like the finished shape of the trailer.
We got the cardboard from a warehouse down the way in my office complex. They are pallet blankets that are taped to gether with
a lot of shipping tape (thanks to Costco).
The proto now lives in the garage while we're working on the real thing. It's a great tool to look at as you scratch your head over how you're going to resolve a problem.