My partner and I have been talking about making some cat paths in our house since long before we owned our house. We recently got 2 new dogs, so now it is a high priority project. The first step was putting up some shelves in the hallway and making a portal from the hallway to the bedroom closet (you can see it just above the tent). The surround for the portal is this 3D printed tunnel
https://www.printables.com/model/3068-cat-door
The next project is going to be putting a portal from the spare bedroom into the stairwell so they can go up and down floors without the dogs being in the way
You can get some premade pieces to add to this, I have one of these which they use to climb up and as a scratching post, and this bed which acts as a resting point along with my standard shelves. Neither were very expensive.