Start by opening the picture where you want to add the scanlines. Then you have to find out how large the picture is, Image > Image Size. Make a new document (Ctrl+N) with the same width as your image and 2 pixel high with white background. Hmm, the width isn't actually that important, the pattern will work whatever width you choose to work on :o)
Now we are going to make the lines. Make sure black is your forground color.
Start by enlarging the image and choose the Single Row Marquee Tool. Click once at the top of the image and fill the selection with black by using the Paint Bucket Tool or ALT + Backspace. The image willl now have one black (1 pixel high) line and one white line (1 pixel high).
Save the new document as Pattern, Edit > Define Pattern. Call it scanlines and hit ok.
Now it's time to add the effect to the picture.
Make a new layer above the background layer (Shft+Ctrl+n). Make sure you are on the new layer and go to Edit > Fill and choose pattern from the drop down menu and then the scanlines.
Hit ok and change the layer mode from Normal to Overlay. You can also try to change it to Soft Light.
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