1. Making the card with sturdy paper.
To make sure your postcard won't get destroyed in the mail, choose a sturdy type of paper, like cardstock. The postcard you make should be about the same thickness as a regular postcard.
2. Cut the paper to the right dimensions.
Take a ruler and measure out a card between 3 1/2 and 4 1/4 inches high and between 5 and 6 inches long.