Create a new file that is 1000 pixels by 1000 pixels
Create a new layer called 'Grid 1'
Select that layer from the layer dialog
Create the grid by selecting the 'Filter' menu then 'Render' -> 'Pattern' -> 'Grid'
unlock the horizontal and vertical settings for width, spacing and offsets.
Use the following settings (image below shows the settings for the 'Grid 1' layer
- Width 0,18,0
- Spacing 21,1000, 2
- offset 0,0,6
repeat for layers 'Grid 2' to 'Grid 7' using horizontal offsets of 3,6,9,12,15 and18 respectively.