I had a general plan in mind, and sketched the benchwork first, to make sure it would all fit and that I bought enough lumber. From there I just laid track using a set of home made curvature templates and a straightedge to assure that I was not fudging things. Occasionally I sketched out not to scale drawings of switching areas and continued to just lay track. This I did more to remember from one weekend to the next what I had planned. After retirement, that wasn't quite as necessary as I worked on it day to day. Of my 70+ industries to switch, I ended up with only one where I didn't quite give myself enough clearance to the backdrop. I had to move it about 1" closer to the switch. It's an angled siding and I might eventually just remove part of the loading dock and awning (on a non-visible side) and position it back where I wanted it.
I had built three previous layouts (practice I called them) in a different location and pretty much knew what would work and what would not work. Through it all, John Armstrong's book "Track Planning for Realistic Operations" was at my side.