I need help for my hovercraft!!!?

I am making a hovercraft for my project. I am using a Styrofoam board and a showercurtan on the bottom of the board. It is not floating as I planned. We don't know where to put the holes on the bottom or how may holes shoud be on the bottom. we ended up making a smaller one that is about the size of a 1x1 square. We put a garbage bag on the bottom of that one. When we turn on the vacuum, the bag poofs up like a baloon and it flips over. Can anybody help me??