Select somebody who doesn’t know the gag and is game for a laugh (at their expense). Ask them to lie down on their back on the floor and place a coat over them so that the armhole to one of the sleeves is over their face. Ask the victim to hold the sleeve upright and tell them they are now in a submarine and this is the periscope of the submarine. If you want to be very elaborate at this point you could give them a mirror attached to a long stick down the sleeve which they can take hold of with their other hand to turn and look around the tube created by the sleeve, but it’s not necessary.
Explain that they are now going to be given a quick course in controlling the submarine and learn a few commands which you get them to repeat after you and will control the boat’s movement. E.g. –
“Hard to Port” = turn left.
“Hard to Starboard” = turn right.
“Dive, Dive, Dive” = Submarine dives under the water.
When the victim says “dive, dive, dive” pour some water down the armhole and explain they forgot to take the periscope down….