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 green or 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.
Print out this zigzag band template on A4 or Letter size paper. Cut out the rectangle then cut along the zigzag line. Take one of the two zigzag sections and use this as the pattern for making the band around the base of your elf hat.
If you went with green fabric for the hat, use red felt or fabric for the zigzag band at the base of the hat, or vice-versa.
Place the zigzag pattern on top of the fabric. Trace around the pattern repeatedly (approximately 3 times) to make a long band that can go around the entire base of the hat.