Step 1

Open any image that you want to apply scan lines to.
Create a new layer by clicking Layer > New > Layer or Shift+Ctrl+N
Give the layer a name, preferably something meaningful E.g. scan lines.

Step 2

On the newly created Layer choose Select from the top menu and then click All.
This will select the whole image using the marquee tool.  Alternatively you can select the marquee tool and drag your mouse the area of the image you want.


 
Step 3

Choose the paint bucket tool and fill the select with white. Now do Ctrl+D to deselect
You should now see a layer that is completely white.


 
Step 4

Still on the same layer go to Filter > Sketch > Halftone Pattern.
This is going to bring up some filters that we are going to apply to the layer.
 


Set the pattern type to Line, not dot. This will generate horizontal scan lines.
You can play around with the other settings and then click OK to confirm the selection.
 


Step 5

Now go to the layer blending modes, Layer > Layer Style> blending Option or
You can select the blending mode by dropping down the menu near your Photoshop layers.
 


Select a particular mode you like, I have chosen Soft Light, but Overlay works particularly good.

We now have an easy way to generate scan lines on your images.