If you don't have a program like Photoshop or Illustrator, just use a program like Word. Choose a bold, thick font, and re-size it until it takes up most of the page. Or you can skip the following steps and sketch straight on the foam.
Choosing a font:
- Print out the template.
- Cut out the template with scissors.
- Trace around the template on the foam with a permanent marker.
The font you use should be bold and thick, like Impact in this example. The concrete won't be very strong if it's less than 1" thick, and even then it will be easy to break, so take that into consideration when determining the size of the letters.