The descriptions of many of the photos in this set tell some of the process:
Step 1: Start with a photo
Step 2: Find edges
Adjust the image with a filter
I used the find edges, edges filter
Since it is a color image, it will find all kinds of cool colors. They're nice, but we don't need them.
Step 3: Convert to black and white
This is found in the image, mode menu. Pretty much the same location for photoshop and gimp. Since this is going to be a silkscreen or cnc image, we will need to clean it up a bunch.
Step 4: Invert
Since this image didn't have great contrast to start, everything looks kind of faint and gray. That can be fixed.
Step 6: Brightness and contrast
This probably can't be done in one pass, so do what you can, hit ok, do the process again. I think I did it like three or four times until most of the background noise was gone. It still wasn't enough to go straight to a cnc machine, and it was enough to hand cut a stencil, so I printed it at this point.