Create a new document, at 15x15px 300dpi with a transparent background.
Max the zoom at 3200%.
Pick the pencil tool (under brush in the toolbar) and brush along the left and bottom side of the document.
You should have something like this:
The go to "Edit -> Define pattern"
Then make a new layer in the document you want to use the pattern, fill it with some color (doesn't matter what).
The double-click the layer,
set "Fill" to 0%.
Go to "pattern overlay", pic your pattern and then you can adjust the size of the grid, opacity and blend-modes.
It might not be the easiest way, but I feel it gives more control over the size of the grid (IMG:style_emoticons/default/8834.gif