Yep, i finally got off my butt and made my grid tutorial that i have been saying i would make. I added a multitude of screenies, so it shouldn't be to hard to follow, but if you do have problems, just post it here and il get back to you as soon as possible.
First of all make a new canvas about 8X8 pixels, and then select the pencil tool (Right click on the brush tool) and draw a diagnal line from one corner to the other as explained in the following screenie.
((Keep in mind this screenie is zoomed in to the max for precision drawing))

Now save this and leave this open and selected, now go to Edit-/ Define pattern and pick a name, whatever will do.

Okay now create a new canvas, lets say 390X140.
Now Make sure your new canvas is selected and go to Edit-/ Fill

Now in the drop down menu select "Pattern"

Now select your newly created grid pattern.

Now click OK and tada, you have your grid.
If you followed everything exactly you final piece should look like this.