Thanks for the ideas. Paper and pencil won't give me a 3D model, so that's out.
I downloaded SketchUp Make by Trimble Navigation Ltd. Google sold it in 2012, I guess. Really nice proggy, especially for free. Very flexible. The free version won't allow construction drawing to be generated (SketchUp Make vs Pro | SketchUp Knowledge Base
), but you get the pro features for 30 days, and I have more than one computer. $700 for the pro version is unreasonable for a single use. They do have a student version for $49, and I have a student that might let me borrow her computer. But $139 for Punch Software might be less hassle. The problem is you get so much with SketchUp for free, it's hard to not use that.
I've been following along youtube videos for many, many hours now. I feel almost like I could hack through the home addition model now, but I'm going to keep watching and following along for a while. I've only watched two different youtube sources, and their ideas and techniques were completely different, so I'm going to try a few more sources before I launch into my project.