You're asking two totally different questions.
For a Trainer, do it in the order that is says on the card.
For a Power, everything that it says to do for the power is part of using the Power.
So, if an Opponent wants to Power Spray and block the power, they don't get to make you flip a coin or choose a target first.
They either block the power, or they don't.
Now, that said, you don't get to rush into the power and then say "too late" when they try to block it.
So, announce the use of the Power, and then pause for them to block it if they wish.
But they do not get to say choose or flip and then I'll see if I use my spray.
That would be too late on their part.