You can do this.. create the 12x18 like you did.. then, open each 6x6. Grab the Move tool and drag it into the new image and let go. The 6x6 will now be a new layer in the 12x18. You can then move it around to line it up how you like. Rinse and repeat. If you need to make the 6x6 a bit smaller to allow for a border, then use the transform tool, hold down shift and drag a corner until the square is resized.. Good Luck1