1. Load the
photo you want to convert in the GIMP
2. Click on Image->Mode->Grayscale to discard the colour info
3. Click on Duplicate layer (button that is circled in the picture below)
4. Click on Filters->Blur->Gaussian Blur
5. Select the X and Y radius as 8.0, and click ok
This is how it will look
6. In the Layers Dock on the right of your GIMP, click on the drop down list against mode: and select Divide
This is how it will look
7. Click on Image->Flatten Image
8. Click on Filters->Artistic->Cartoon
9. In the dialog ox that opens, adjust the mask radius and percent black that you want in your sketch, I have shown the settings I used in the picture below. Play with the sliders to study their effects.
This is what you will get