Creating the pattern

To make a successful pattern, the size should be relatively small, and the background should be transparent
Example:

So create a document with those settings, and get the pencil tool

Make sure the settings for the pencil tool are set like this:

Zoom in on the document to about 800% or 1600% even, then draw a line from the top right to the bottom left, like so

To define the pattern, hit CTRL+A (not necessary) and go to Edit > Define Pattern

Using the pattern

Open the image you want to add the pattern to, anything will do:

Create a new layer for the pattern then get the paint bucket tool

Set these settings for the Paint Bucket tool:

Click the canvas

Invert (CTRL+I) and change the layer Opacity/Mode


That's it for the basic scan line, hope you learnt about patterns!