Well done for being prepared in asking the question before the game, and apologies my input is too late but one for future reference in any penalty situation (including in game/not shootout)...
I learnt from bitter experience one piece of advice: as the player taking the kick walks towards you make the first thing you do tell him 'on the whistle pal'.
They're nervous, so may forget this vital piece of information, and if they do it could leave you in a tricky position (as it did me).
My first shootout caught me out with this. Claret & Blue v Red.
Score at this point is 2-1 to Red (who took first kick) and each kick taken up to this point has been converted. Up comes Claret to take their second kick. I note his number, check the ball placement, walk backwards into my position into the Claret goalkeeper, who is right behind me instead of where I had told him to stand near the corner flag opposite to me.
I turn around to tell him to go where I told him originally to hear the taker sky the ball into the trees behind the goal. By a mile.
As I hadn't seen the penalty being taken I didn't know if it was legal, so I told them to retake, which they then scored and Claret went on to win the shootout.
I hadn't actually done anything wrong myself and I was correct in law to retake but it put me in a very difficult position and naturally red were unhappy, as you would expect. The thing that did get to me though was it was entirely preventable by reminding them it was on the whistle every time a taker came up.
I told this story to a close friend and colleague a couple of weeks ago, who said he'd never had a shootout and he kind of laughed it off. He text me last Friday to tell me he had a cup game the next day and I reminded him of this story and to order each kicker on the whistle.
It did go to Pens, he used the advice and agreed it had made life much easier.
Please learn from my bad experience, I felt a right tw@t!