How to add a square grid as a layer?

[Larb]

1) Create a new image which is the size of the squares you want. so for a 50px grid, make it a 50 x 50 image. Make sure it has a transparent background.
2) Press ctrl+A to select all. Go to Edit -> Stroke. Select "inside". Set the width to how thick you want the lines to be.
3) Go to Edit -> Define Pattern. Name it appropriately.
4) On the actual image you want to put the grid on, create a new fill/adjustment layer. Pick pattern. Then select the grid pattern that you just made.
5) Profit, etc etc.

You can make isometric/orthographic grids this way too. Although it's slightly more fiddly.