One more great option using the grid stamp and template I've shown below is to first stamp your grid onto a piece of white paper. Then carefully place the template over the grid so all nine squares fit evenly inside the dotted lines. Secure at the top with a piece of tape.
For this card I used a stipple brush to add some background colors before stamping my flowers over the template
When you are done, remove the template and you once again have that great postage look.
I added a sentiment with rub-ons after removing the template.