Paper cups are handy and easy to make, though you shouldn't use paper cups for hot beverages. You also should drink from the paper cup immediately before the liquid is absorbed through the paper and starts to leak. Paper cups are useful, though, for quick drinking needs.
Cut a piece of paper into a square.
Fold the paper from one of the corners in half to form a triangle and crease it.
Hold the triangle so the 90-degree angle is facing upward. Take either corner and bring the point to touch the middle of the opposite side of the triangle.
Fold the other corner to line up with the fold you just made and crease.
Separate the two parts of the triangle that has formed at the top by poking your finger in the middle and folding each part down to the opposite side. Crease. You will now have a perfect trapezoid.
Lightly pull the two sides away from each other to open the cup.