I built a small garden shed our backyard. The intent was make it look like the main house as much as possible and still maintain its functionality. These are the steps I took to make my vision a reality. While this is probably more work than required (most people don't need a stucco garden shed with 15 windows and 3D printed molding) the same basic steps would apply to a simpler design.
Tools You Might Need:
Most of the design work for this project was done in a 2D CAD program. I used DeltaCAD, its free!
I started out with a "plan" set of drawings and then produced an "as-build" set of drawings as the structure was being built (over months). This let me be more flexible while I was building because while I had a plan, I didn't need feel obligated to stick to it at every last joint
The "as-built" set of drawings let me make decisions about the design based on how it was actually built and not based on a plan I had deviated from.