well? PLEEEZE?!?!
You don't.
First off, in your brick inventory, look at the last bricks, called print bricks.
place a couple of these.
Than use the printer in your "Q" inventory to shoot one of them.
after than either choose a decal or a letter to be put on the brick.
a faster way is once you shot the brick, just hit the letter (you want it to display) on your keyboard.