Firstly, you will need to open a document, in the following format:
Click File>>New>>


You should now have a document similiar to this:


Fill in the following rows to create either the pencil tool, in black colour.

Vertical:
Horizontal:
Diagonal:

Then select the document, and select EDIT>>DEFINE PATTERN.

Then open the image you wish the scan lines to be on, and select EDIT>>FILL and apply these settings: (I'm using a diagonal scan line)


After that, it will seem to dark, so simply change the opacity:
>>
(Use between 30-50)

Final Result: