Okay to start things off I’m going to show you how to turn this into this. See the difference?
Okay to start off you are going to start a canvas with the following settings
Alright now that you have you canvas down you’ll need to zoom up so press ‘z’ on your keyboard and then at the top right hit ‘fit screen’.
Your canvas should now fill the screen. You’re going to want to right click the shape tool and hit the ‘Line Tool’ like below
Okay now we are going to make a straight line from top right to bottom left or top left to bottom right. What ever fits your fancy it will work both ways. with the thickness/weight of 3-5. If you zoom back out it should look like this
Okay now we hit Edit at the top next to File and 3/4 down the drop menu should be ‘Define Pattern’ hit it and then name your pattern. Click okay.
Now go to your project which you want to add the lines and create a new layer, CTRL, SHIFT and N. Now grab the Fill Bucket Tool and make the layer what ever color you want because you wont be seeing it. Okay now on your layers tab at the far right side of photoshop highlight your new layer and change the Fill percentage to 0.
Now right click your new layer hit ‘Blending Options’ tick the Pattern Overlay option and hit the drop down arrow next to Pattern: and the pattern you defined earlier should be there.
Now it may look like it doesn’t tessellate very well but when you scale it down a lil bit and change the opticity a bit and it will look perfectly fine. You can do this with other shapes as well such as tessellating octagons. http://gwydir.demon.co.uk/jo/tess/hexagon.gif