I pull the base loose from the cup
Hold the flag down
Stand on the side where my shadow doesn't cross the line of putt
Other hand is hidden behind
I try to hold the flag vertical and not slanty despite the factor I have the base loosened
I stand still until the putt is well on the way
For Pros, I suspect holding it to the very end is to eliminate the distraction of a body moving while the putt is rolling - these guys watch the putt all the way to get more input...