Step 1: Materials
-Black felt or fleece fabric, MUST be fairly stiff to stand up a bit.
-gold felt or fabric, contrasting felt for letter H (Could also use house colors, e.g., green and silver in an S for Slytherin)
-Tailors chalk, or other light colored writing tool
-needle, or sewing machine
-hotmelt glue or more thread to attach the crest
Step 2: Measure and Calculate
You'll be cutting out a piece of fabric a little wider than a quarter of a circle.
Take the measurement of the circumference of your head, multiply it by 4 and plug it into the equation for finding the circumference of a circle (2 pi r=C).
To see how I did this, check out the graphic:
(it also has the awful sketch from the hogwarts crest.)
Take the measurement you get for R the Radius and add an inch or so for your seam allowance.