I bought the shirts at Old Navy (on clearance) and then cut up one of Rachel's Guarantee Girls cleaning shirts (Larry says that's very "sustainable" of me). I used Old English Text MC and made the letters about 3" tall and traced them, reversed, onto Wonder Under.
I used this shamrock from a blog (and I can't find it! Rats! If you know where I got it please tell me) that I had saved to my images folder. I made it into a guitar. First I ironed on the letters and then I top-stitched around everything and put the knobs and strings on the guitar.
It took longer than I thought to do all that sewing, but in the end, I'm pretty happy with the result. I like it so much, I'm going to make a pillow ... sham.