You have to stick something up your butthole. A finger, toys, something…if you’ve done that and it didn’t do anything then it simply may not be for you.
My advice is to keep trying. It took me a while too, but I've found the right toy (and we've tried a lot of them) and now I get a pretty good milk. Not shooting cum everywhere just yet, but definitely get a milky stream leaking out of my cock.