The critical thing you’re going to want to keep in mind for ethernet wiring is getting everything wired back to a central location is key, and spending the extra time and effort to do home runs instead of having multiple junctions can save you a lot of headaches down the line. That being said, you can run more than line at the same time.

For where to start, deciding where you’re pathing to and how you’re getting there is what you want to focus on. If there’s no existing ethernet, what about existing fiber coming into? Or is there a central block where all the RG59/6 terminates? Even just a nice shelf near the furnace can be fine if the furnace doesn’t cook the room its in. Is there a crawlspace or attic where you can run all the wiring through?

Getting answers to those questions are where I would start. I would suggest though that retrofitting ethernet wiring is a project that you might be better off waiting on until you’re also doing other work in the space(s), and you can get away with a lot just using low profile cables and tacking them around doorframes apartment-style in the short to medium term until you’re ready to do it all at once and/or need to do related work (like if part of the building’s power lines need to be replaced or you decide to get new flooring or whatever).

EDIT: Well, my notifications went rogue and said this was a new thread lol. Leaving the above anyway.

I bought a 1959 and wish I had gone further with my remodeling than I did, and I took out all the floor down to the studs in the main level and filled everything with soundproofing insulation before redoing the subfloor with 1 and a 1/4 inchs of OSB that I glued and compression screwed to the joists. One thing I’m very happy I did do was run Ethernet to most rooms while the floor was open, having all go to a central hub in the utility room where they’re connected to switches, one for the upstairs network, one for the basement network since I also made the basement a separate apartment. Long way to say I wish I had planned out every repair and remodel and upgrade I could think of before ever touching a hammer so I could just do all the demo and remodeling at once. Now I’m stuck wishing I could go back and redo all the old electrical and plumbing before I had done all the finish work. Learn from me young one!

if there is any coax already ran, MOCA is pretty reliable and robust