Tnnaroot, you already started step one. First off are you handy with tools, are you local to a hardware store or metal supply, do you have any power tools. For a do it yourself project You'll most likely need a drill, angle grinder, cutoff wheels, grinding wheels and maybe chopsaw. For one thing, you would have to be some what mechanically inclined either in understanding the visual perspective of the project or hands on. If you don't have any tools and or not close to a Harbor Freight Store, then your probably better off just paying someone on the site to build you one. Beaudry is selling a nice compact one on Ebay that you might be able to bid on or make him an offer directly. In the long run you might be spending more trying to Do It Yourself then just to purchase one already built. Plus there is a big probablity of getting hurt if your not very mechanically inclined and or don't understand leverage and torque.
Tnnaroot, By no means am i trying to discourage you from starting your own project. Im just trying to fill you in on what you have in store if you do. But to be asking how to go about each set by step then you are kind of asking alot because everyones designs are unique and take different steps in building. Take your time to do your research on these trailers and learn about the concepts by going through all the push trailer and 169cc and 79cc engine threads. If u do decide to start your own build then figure out what size motor you want to go with for a start. Lower center of gravity is always better on a trailer design. There is alot of good info on which parts to buy and where to buy on this site. Good luck and be safe.