This T-shirt will give the impression that you are carrying a powerful handgun and protect you from all but the most determined mugger.*
This is a overly optimistic view; Using this while being mugged is likely to increase your chances of being fatally wounded quite dramatically.
Step 1: Find an Image.
First find an image you are happy with.
I used google image search.
Step 2: Print Image.
I tinkered about with the image on Photoshop first to get the lighting and size to my liking.
I printed two on one sheet so that I could choose the best result.
Step 3: Iron on Image.
Remove the paper backing.
Step 4: Add Straps.
I chose tape for speed.
Step 5: Put on Your Coat.
"Are you talking to me?"