I use graph paper, for the most part. I square on said graph = I wall panel. If I use a floorplan from a magazine it gets tricky, but keeping with the notion that 1 square = 1 wall panel, you can get a reasonable facsimile. Once you tweak it in your game to best represent the floorplan, transfer that to graph paper.
I was dismayed to realize we didn't have graphs in build mode in Sims2. Someone had recreated the Wright Way house for Sims3. I downloaded it and then decided I would build it in Sims4. I went back into Sims3 and copied it panel for panel onto graph paper. There were a few anomalies in this rendition, so I went back into Sims2 to take another hard look at the original. From there, I fixed what was wrong in the remake and built that into my Sims4 game. (I had it in my head, I would enlarge the house in Sims4. However, the grid isn't the same as in Sims3. The house is already ginormous!)