Yeah, this will show you how to make those grids. It's kinda played out, but to the point where it's an accepted as common usage rather than a technique.

First make a new project according to these settings.

Right click on the brush icon which is located on the tool pallet. Select Pencil Tool from it. Set the pencil brush to the smallest, so it is one pixel wide.

Zoom in about 1600% then created an "L" shaped bored on two of the adjacent sides. Then select the whole image.

Go to Edit > Define Pattern and then name is something like grid, or whatever.

Now open up an image you want to put the grids onto. Then right click on the gradient icon located on the tool pallet, select Paint Bucket Tool

At the top you need to change Foreground to Pattern then select your pattern, the one you just made should be there.

Not go to you Layers Pallet and create a new layer above the one your adding the grid to. Set the opacitcy down to around 40, or whatever you desire.

Now you just fill the image with the Paint Bucket Tool.

If you find this information inaccurate or have questions post in the forums