make a new image...
size doesnt depend but dont use too small...
make a new section use rectangular marquee tool hold shift and draw a rectangular, then contract the section.. say 2 pixels, del the section, and then move the section upp and left 2 pixels, then define pattern, call it: say grid system.
And then to layers create a new layer, use paint bucket tool choose fil the pattern and chose the grid system and then fill the layer... and you're done...
I hope u understand this... my poor english