Diagonal scanlines are like horizontal scanlines, but at a 45 degree angle. It is good for making backgrounds. This method is similar to my horizontal scanline tutorial.
Create a document that is 4px wide and 4px high. Select the pencil tool and set it to 1px. Draw a diagonal line like in the picture(the color determines the color of your lines). I zoomed in a lot so you can see. I suggest you do the same. Go to EDIT>>DEFINE PATTERN. And press OK. Close the document.
Make a new document. Fill it with a solid color of your choice. Press CTRL+A to select all and then go to EDIT>>FILL. Under "USE" Select Pattern and then find your diagonal that you just made. It's under "Custom Pattern". Select it and press okay. Your fill should now be filled with diagonal lines. Reduce the opacity so it's not too dark. Have fun!
Final Result