It all depends. When they are little you must follow them whatever they do and immediately use the squirt bottle when they attack the leather. The secret word is "immediately" because they must think it happens in one thing "Me scratch-water comes" Then they'll learn quickly to leave the things alone.

The claws of F+T are not trimmed (because I prefer to keep my arm) and the only things they attack now are things where we have not been consequent enough when they were babies