Put everything in one big pile in the middle of the room. If you have a lot of stuff it’s going to look and feel insane—this is good because it’ll make you aware of how much you have.
Sort everything into three piles: a pile for yes, a pile for maybe, a pile for no. After you’ve finished go through the maybe pile and be critical, discerning, decisive, erring towards no as much as possible. Once you’ve gone through the maybe pile and have your two yes and no piles, put everything from the no pile in bags or boxes to give away.
Find a place or devise a solution to store everything in the yes pile in an organized and clutter-free manner where it’s tucked neatly away. If you can’t find a place for everything you either have to get more creative or get rid of more stuff!
Then make a habit of putting everything away when you're finished using it and cleaning a little bit every day as maintenance.