This will be a quick tutorial because the steps are very similar to my tutorial for messages on a canvas.
1 16x20 poster
1 set of 2inch vinyl letters (available in the hardware section)
1 can spray-paint in the color of your choice (be sure it is a color different enough from the colors in the poster that the words will stand out!)
Place the letters on the poster in the message of your choice. Use a ruler to make sure they are evenly distributed and straight. I placed my letters 3/8ths of an inch apart with a 1inch space between words. There is also a 1.5inch space at the top and bottom of the poster, and 2inches between lines of words.
Spray-paint the poster with the color of your choice. Let dry.
Peel off the letters, and frame to enjoy!