I do it in Paintshop Pro, although it is possibly to do it in Photoshop, I think. In PSP, you make a new layer, select the grid under textures (usually it's set to no texture, but you can pick one), flood fill, and and then adjust the opacity on that layer so it looks how you want it to. Then I erased the bits of grid where I wanted the whole pattern to show through, or where it fades out on the left edge.