Simplest solution: Start with a large cardboard tube of the kind used as a mold for porch footings (available from hardware store or home center). Cut holes in sides. Cut internal shelves to install just below holes, leaving a space at the back of the hole in case the cat wants to go up the inside, and cut a platform to install across the top. Cover everything in surplus carpeting and assemble with glue and screws. Or build some other form of sturdy shelving, probably out of wood, and similarly cover in carpet. If you look at the ones in the pet stores and think about those materials, it should be fairly obvious how they go together. Don't be too surprised or disappointed if the cat decides it's less interesting than you thought it was, especially an older cat.