...*waving hello* ...
so do you have a picture in your mind of what you want it to look like??
2 wheels or 4 ???
What sort of driving do you want to do with it .. and what horse will you be hooking it up to??
... for me .. the answer to Every Thing is "google"
I would try something like .. "horse drawn vehicle patterns" or "building your own horse cart" ... see what comes up!!
I wish you the VERY Best Success in your endeavor!!
~ Lyrical Lass