Similar to others who have posted, I have never really had an issue with my cats scratching what they shouldn’t, and I am certified cat lady (lots of cats past & present).
I have quite a few small (size of a big placemat) sisal mats strewn about, plus two nice tall scratching posts with sisal rope. They like to stretch, so I try to have at least one tall or angled option for them.
Scratching posts with carpet may encourage scratching of other carpets…the ones on your floors. Burlap, jute, sisal have been preferred by my cats, and look nicer, IMHO. I just know sisal floor coverings for me would never be an option, since they’ve essentially been trained to use it freely for their pleasure. The cardboard scratchers are also a favorite.
I show them what I want them to scratch, I sort of rub it with their paw, & from there they kind of figure it all out themselves. When I have had a kitten (which I try to avoid - omygodstop), I may have to redirect them just a few times at most.
I try to put the mats in places where they would be when the urge to scratch hits. For example, when I get home one of mine gets excited & likes to scratch, so there is a mat floating around in the entry way. They congregate in the kitchen when I am cooking, so I have small mats or cardboard scratchers in quantities equal to my cat numbers (kitties like to lie on the mats/cardboard in “loaf” pose - this way they don’t have to share). They all play raceway down the long hallway, so there is a tall post on one end & a mat on the other.
I haven’t had an issue, as I said, but if you do encounter a problem scratcher (my friend inherited one), that is resistant to normal positive efforts, there are still things you can do. Double sided tape on the corner of the couch she likes, or other stuff that doesn’t feel good, as a deterrent.
Soft paws are also an option. They are soft gel nail caps and come in fun colors.
I keep my cats nails trimmed. They are 100% inside cats and I hate feeling sharps when they sit or climb on me. You should at least watch the dewclaws on the front. They can curl back in and create problems