Open a new document. 12x12 pixels, transparent background. Zoom in and using the pencil tool with a 1 pixel setting, draw a line on the bottom, and one on the far right so it looks like a backwards L. Then go to Edit > Define Pattern. Name it Grid or whatever you want. Then when you want to use the grid on a graphic, go to Layer style > pattern overlay, and select your grid. That should work. If not, I haven't explained it right. This is just off the top of my head.