Fold the other top flap backward, so that there is a flat top to the cup. Open it up by sticking your finger between the top flaps.
The remaining flap on the top should be folded behind the model. Put your finger between those two flaps and push on the edges of the model. The cup will take shape.
The finished model holds water, dirt, seeds, sand, or anything else that might slip through your fingers. With a little work you can make them square and flat on the bottom, which will help them to stand up on their own.
I look for shiny paper to use for origami cups, since water takes a little longer to penetrate the surface. If your paper is too thin to hold water, fold it into quarters before beginning.
Remember.... drink responsibly.