Currently using GIMP 2.8.16.
Duplicate the image/layer.
On the top-most layer, go to
Colors - Desaturate - Lightness
On the same image/layer apply
Colors - Levels - Edit these settings as Curves
The idea here is to add a slightly darker contrast to the image. Not too dark, or not too light. Please note, if you make the levels too dark, it will result in lots of artifacts (shading) displaying in the final result. If you make it too light, the lines will be faded, not dark.
Duplicate the top-most image/layer.
Invert the top-most layer Colors - Invert.
Change the layer mode to Dodge.
Go to Filters - Blur - Gaussian Blur and set both Horizontal and Vertical to 15.0 px.
Edit Copy Visible the top resulting image/layer, and paste as a new layer. Change the layer mode to Burn. Save the results of the final step.