GIMP Digital Screentones for Manga & Comics

This should help in creating and applying screentones using simple
patterns like lines, circles, diamonds, euclidian dots, and crossing
lines. 

Video: https://www.youtube.com/watch?v=Ebwy2J9URe4

1. Have your comic ready for adding screentones

2. Add a transparent layer and call it "screentone" or something that is
   easy to remember.
   
3. Select the area(s) you want to screentone using one of the select 
   tools. The "Fuzzy Select Tool" may be of some use if you have
   "Sample merged" enabled under its "Tool Options."
   
4. Fill the selected area(s) with a light gray color while on the 
   screentone layer.
   
5. Image --> Mode --> Grayscale

6. While the area(s) are still selected, use 
   "Filters --> Distorts --> Newsprint..." HOWEVER, you can also have
   none of those areas selected and change the "Blending Options" Mode
   to use "Overlay" and achieve a similar effect if you are still on
   the screentone layer.

7. Ajust the Newsprint filter settings as needed and click Ok.

8. If you used the Rectangle or Ellipse select tools instead of the
   Fuzzy or Free select tools, you should then adjust the screentone
   layer's opacity to around 70% or lower and then erase what you do not
   want to keep.