Take a rectangular sheet of paper
Fold it in half lengthwise
Fold it in half again, then unfold
Fold the top corners down to the center crease
Fold the bottom flap up on one side
Flip it over and fold the other bottom flap up
Open the bottom edge and flatten it into a diamond shape
Fold the lower edges of the diamond upward on both sides
Open the hat from the bottom and shape it to fit
Adjust the folds as needed to hold its shape