Demo

Direction

1. File > New > 1x3 pixels transparent background.

2. Zoom in to 1600%.

3. Press D.

4. Go to Pencil Tool, use 1 px diameter.

5. Spot on your document like this.

6. Edit > Define Pattern... > Name it scanline (or anything you like) > OK.

7. Open your image that you want to add scanlines on.

8. Go to Paint Bucket Tool > Fill with pattern > choose scanline pattern.

9. Creat new layer (click on Layers palette) > set Opacity to about 21%.

10. Click on your image.

Note In step 9, you can increase or decrease Opacity as you like. You also can choose other color for scanlines. :-)