You right click it, and you'll get 10 minutes to create whatever you want inside a 50x50x50 space I believe. And once those 10 minutes are up you won't be able to place anymore blocks, my best suggestion is to plan out your build and time it to make sure you have enough time to make what you want to make. You can't place minecarts or paintings, keep that in mind. Check out your favourite /pw's and see what they have that you'd like. /pw's are temporary, they will be there for a full season, but after season resets, they are gone.