Iron them on or sew them on. A lot of crafts stores (also walmart and target) sell letters you can iron/sew on. I would check with the craft/fabric stores first since I'd assume they would have a better selection.
Sew them on. Get a fabric that would look nice on your hoodie. Cut out letters. Use string that's the same color or compliments the fabric well. Or find beads that have letters on them and sew them on.
Iron on. (Make sure you get either the lighter or dark fabric kind depending on your hoodie) Or you can make a stencil out of freezer paper, and get fabric paint to paint them on. Freezer paper works well on a budget because you usually have it in that house and the cost of fabric paint is a lot less than good quality iron-ons.
At Walmart and craft stores they sell iron-on letters. They're easy to apply and come in a variety of fonts and colors. You could check those out.
Also, at the very least, they make material markers with this gel in them that can be applied to clothing and stays on.