You want to start out by opening the image that you want to add the scanline to, I chose the Lightforce wallpaper and scaled it down to 300×225.
Next, create a new layer 
above the background n the layers from palette. Then go to the solid palette and change the Textture to “Line-Horiz 4″ and set it up similar to the image on the left with opacity near 60%.
Rectangle tool from the toolbar and draw a rectangle over the entire image on the new layer. Finally, on the Layer palette, change the opacity of the layer which contains the scanlines to 25% and you’re done 
