A TRANSPARENT GRID TO OVERLAY A PHOTO WITH 1" SQUARES

Step 1) File>New width: 73px -- height: 73px -- Contents:Transparent

Step 2)Select>All
   Edit>Stroke width: 1px -- color:black (or your choice) -- location: inside

Step 3) Crop Tool (options are on the toolbar at the top of the screen)


Set the options to: width: 72px - height: 72px - resolution: 72px

Start drawing your crop from just outside the bottom right corner, moving up and across the canvas to the left.
like this
(This will trim a 1px border from the top and left sides of the image, allowing the grid sections to correctly overlap on the grid.)

Step 4) Edit>Define Pattern - OK
(This will store the pattern and you will be able to use it again.)

Step 5) Open up the image you want to grid over.
Layer>New Make a new layer
Select>All on the new layer (Layer 1)

Step 6) Edit>Fill
Then select: Use> Pattern
Custom Pattern: Click the down arrow to see the patterns available - your new pattern should be the last one. Select the pattern.
Select>Deselect
If you have done it correctly you will see something like this:

The pattern will always be available to use over again. You can modify the size of the grid by (Step 1)changing the dimensions of the new file and (Step 3) setting the crop tool options to one pixel less.

You can also make the lines wider or narrower by changing the stroke width (Step 2)
(You figure the math!)