>>>see visual #1

Open a new file (Width: 1, Height: 2, white backround)

>>>see visual #2

Press 'D' on your keyboard to make your foreground color black and your backround color white.
Zoom in the canvas as much as possible (1600%) and use the pencil tool (1 pixel in master diameter) to make the first half of it black.

>>>see visual #3

Go to Edit > Define Pattern.

>>>see visual #4

Name your pattern whatever you want (I chose to name it Scanlines #1. Makes sense, yeah?) and press OK.

>>>see visual #5

Open the image you want to edit and create a new layer.

>>>see visual #6

Set the layer mode to Soft Light.

>>>see visual #7

Select the pattern tool and click on the pattern that you just saved.

>>>see finished result

Cover the desired areas with the pattern, and play around with the layer Opacity to your liking. You can also change the layer mode to overlay, colorburn, etc.

You can create different patterns, such as grids and diagonal scanlines using the same steps.

>>>Grid visual #1
>>>Finished result with grid

>>>Diagonal scanlines visual #1
>>>Finished result with diagonal lines