Seal the clear PVC pipe piece into the end of the garbage bag or weather balloon using Duct Tape.  Place and spread the bag on a flat floor or large table and cover it with the flexible foam board.   Tape at least two edges of the foam board to the garbage bag so that it doesn't slip off the garbage bag during inflation.  Have someone sit on the board and then turn on the blower. It is easier to maintain a centered balance on the board during inflation if your legs hang off one edge of the board.  The increasing air pressure inside the bag will lift the person several inches off the floor.  Have several pieces of Duct Tape ready to seal any holes or rips made in the bag when doing the demo. 

If a second person is used as a spotter for this demo it will be more convenient if the spotter uses the foot switch on the vacuum blower which will give better inflation control.