I've done it. It's not hard.
I'm sure it would be impossible in a real fight. But if someone is just squirming around, it's still pretty easy to put em in it. All you do is step over, turn, drop to one knee, put the other one in their back to turn them over the rest of the way, and it's on.
Although in a real fight, i'm sure someone would just grab your leg...then you got no chance.