Requirements: Wavelet Denoise plugin: http://registry.gimp.org/node/4235
Shared earlier a sketch that I did on another forum and thought, well, I should share more of my secrets with you all. I actually concocted this effect a few months back and have used for various sketches since then (again other forums I frequent). I usually add other filters to the mix such as Impressionist plugin and such, for enhanced sketch like result, but for now, this result only uses the plugin so you can see the full effect of this technique.
1) Open target and then duplicate layer.
2) On top layer run the filter with the following settings:
3) On the base layer run the filter with the following settings (alternative method is to just open the image run the filter with the above settings dup and do so again with the new layer, but that's the lazy man's approximation):
4) Set Top layer to Grain Extract then flatten.
5) Autolevel then desaturate. Simple as that.