This is one of the most popular requests I get, apparently I am known for my scanlines & everyone likes them so I am making a quick tutorial on it.
Firstly, you need to download some scanline patterns.
After you’ve downloaded em, extract them to your pattern folder, for me it’s in C:\Users\Name\.gimp-2.8\patterns after that if you have GIMP opened, close it and restart.
To use them now, you open GIMP & Select the Paint Bucket tool when you’re done, use the option Pattern Fill and select one of the downloaded patterns and use it on a new layer (Presuming you know how layers work). I like to set the opacity to 50% & layer mode on Overlay, when you’re done it should look something like this on an image.