Note: This tutorial was done using Adobe Photoshop 7.0
Open a new window with:
Width:150pixels Height:150pixels
Resolution: 72 pixels/inch Mode:
RGB color Contents:White
Go to Channels Pallete Window > Show Channels
and make New Channel as shown on the left.
Next make a Fixed Selection with Rectangular Marquee
Tool (M) 15X15 pixels and make a selection. Paint
it with white.
Rectangular Marquee Tool (M) still selected
move the selection 1 pixel to the right and 1 pixel down [do
it with arrows on the keyboard]. Press Delete.
Now go back 1 pixel up and 1 pixel left [come to the previous
position]. And go to Edit > Define Pattern,
enter the name and press OK. Deselect it,
Ctrl + D.
Make a new Channel again like shown below.
Select the Paint Bucket Tool (G)
And select your pattern from Options Toolbar
Paint it with your pattern, you should have same like on the
left.
Now Go to Layers palette and click on the
Layer 1. Choose the color for your grid.
Select the Paint Bucket Tool (G) and don't
forget to select Foreground for Fill, from Options Toolbar.
Next paint it and Deselect (Ctrl+D)
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Open a new window with: 
Next make a Fixed Selection with Rectangular Marquee
Tool (M) 15X15 pixels and make a selection. Paint
it with white.
Rectangular Marquee Tool (M) still selected
move the selection 1 pixel to the right and 1 pixel down [do
it with arrows on the keyboard]. Press Delete.
Select the Paint Bucket Tool (G) 
Press Ctrl button and Click Alpha 2 channel.
(Ctrl+Click Alpha 2)
Now Go to Layers palette and click on the
Layer 1. Choose the color for your grid. 
And finally what we have is thin Grid Lines!
