Here's what you need:
Iron: $13 if you don't have one
Patches: This is the exact one I used, set of two for $6, but you can find almost anything on Etsy.
Shirt: Again, sky's the limit, I used an Everlane silk blouse, and this one for Bit but you could use something you have, or find something vintage.
Pressing Cloth: I used a microfiber lens cleaner that came with my sunglasses, but anything will do
Position the patch in desired spot. Remember, check twice glue once!
Put the pressing cloth on the patch. Press the iron on the cloth over the patch and hold for 20 seconds.
Flip the shirt inside out and press the iron directly against the fabric.
Literally, that is it! So easy, right?