There are a few ways to overlay letters. One way is to use a transparent sheet of paper or plastic. Place the sheet over the top of the letter you want to overlay. Trace the letter with a pen or pencil, then remove the sheet and write your own letter in the same space. Another way is to use a computer program like Photoshop or GIMP. Open the image of the letter you want to overlay in the program, then create a new layer.