Tape your triangle to a flat work surface. Tie a pencil to a string and stretch the string taut from the apex (or tip) of the triangle until the point of the pencil touches the midpoint of the triangle's base.
You'll find that it will make the next step much easier if you tape the string to the apex of the triangle as well.
Prepare a large sheet (at least 22 x 14 inches) of red felt. If you want a soft or droopy hat, use thin felt or a soft-type fabric such as velvet.
In case you don't have a large enough sheet of felt or fabric, you can use two smaller sheets. Each sheet should be the same size or a little larger than your hat pattern.