A friend asked me recently how I draw up a plan for my furniture. Just like in landscape architecture and construction, a well vetted plan can help identify early pitfalls.
I always start with a few simple sketches for form and proportion. I'm always doodling and I think it's important to capture these off the cuff ideas. You just can't draw enough.
Once an idea starts getting firmed up I will draw it in AutoCad. This allows me to get inside, see the critical dimensions and really plan out the individual parts. It is super nerdy, but I get a kick out of the precision.
At this point I also create a 3-D model in SketchUp. With a model I can anticipate how different parts will interact visually, verify massing and see how parts move - like doors and drawers. I often go between the Cad plan and model making several revisions to each.
These plans end up living in the shop. They collect notes on dimensions, ideas and more doodles.
After all of that I build full size prototypes, but that's another topic.