This is an easy tutorial for scanlines.
Firstly, open up a new image. Make sure the size is 4×4 pixels, and the image must be transparent. Next, click the Zoom tool,and zoom in by 1600%.
Now, select the Pencil tool. Right Click the Brush Tool and select the Pencil tool.Be sure that it is on a 1px radius, and click the tool until it looks like this:
I used the color value #000000, but you may choose which color you want your scanlines to be, but #000000 is recommended.
No go to Edit>>>Define Pattern then call it scanlines, and you are done!