Arrange and pin the pressed letters onto the hoodie. Affix with whatever stitch makes you happy. I use a modified
ladder stitch that's similar to a blind hem. Basically the part of the stitch that's showing is perpendicular to the edge of the letter, then I carry the thread up through the fabric of the hoodie to the next stitch location, which makes the thread hard to see when it's done.
You may also use fabric glue, but I'll call you a wuss. =]
Tie off and tuck in your thread end, or let your glue dry, and then commence the FAT.