Tear, after scoring, or cut off the extra piece.
The fold is at the bottom of the triangle in the picture below. Fold the left hand point over.
Repeat on the other side.
Fold the first top flap down.
Turn over and fold the other top flap down.
Spread the cup open at the top. It will survive a good drink of water, but then it will get soggy.
Now, have a drink of water!