How do I keep cats out?

For the couch and furniture, you can try double sided sticky tape on the areas where they scratch. As for the curtains, you could try the tape on them also, and get a cat tree or cat napper to put next to the window, so they won't try to climb the curtains to get up to the window. As for the reaching under doors and other things, I'm not sure what to tell you. The best advice I can give you is to get a water bottle and spray them when you catch them doing stuff you don't want.