I start with a lined yellow pad, and draw very roughly with a pencil. It is more of a mental exercise in figuring out how each of the joints, and the overall design is going to come together. I pencil in sides, top, and any particular areas like a drawer, or internal braces, etc. Then I take these rough drawings, and convert it into computer design. I have been using a draw program, with simple boxes, and lines to more exactly determine dimensions of each piece. I often take scrap pieces from the shop, at different sizes to see how they feel in terms of proportions with one another. No need to build the whole unit to do this. A skirt for a table for example can be quickly judged.
Once I am done with the computer, I have a complete drawing with each piece specified, with a material and cut out list. I use the construction master calculater, which figures in fractions, and see what I need in the way of material, plus about 25 percent for waste.
My drawings are not to scale though I would like to learn Sketch Up.
I spend a good deal of time in design, then the actual build flies. I can concentrate on building procedures.
It takes time to do design work. For example I did a set of doors for a customer, and spent 4 hours in design, and then I built them. I could have built 8 in only a little more time. Design once, build a thousand.
Anyhow I respect the fact that someone who has been doing woodworking, or building for a lifetime, can do a rough scratch, and start building. I am not that experienced. Besides, I love the design stage. That is pure creativity. As I am envisioning how the pieces are going together, and what kind of joinery I am going to use, I get a rush out of that. I come up with whole design systems, and how they can work together, and then when I see how other designs/systems do it, I can better appreciate their thinking, since I am so embroiled in the nature of what is needed. It's all a labor of love for me, so no matter what part I am doing, it's a plus.