I marked the uprights so I’d have some reference to fasten each level of the cat tower. Then, I assembled it in its entirety to check for fit prior to upholstery. What gets tricky is every piece gets ½” larger after upholstering. The carpet is about ¼” thick. I made up for that by cutting my boards a bit smaller (cut list is accurate), and trimming away some carpet in some locations on the uprights. You’ll see where in the next few steps.