This tutorial is very similar to the scan line tutorial you did before (assuming you did it)

First create a new document that's about 10 x 10 (Or if you want a smaller grid use a smaller size) and zoom in all the way.
Again, use the pencil tool and make a line on the left and top side of your document. Something
like this:

Go to Edit > Define Pattern... and name the pattern whatever you like (Preferably Gridlines)

Next open the image that you would like to apply the gridline pattern to. Then go to Edit> Fill and use the same settings
you used before on the scan line patter tutorial.

Another 3 step tutorial and then your done. Again, if you would like to add a little more flavor to the pattern you can
change the color of it.