It’s really simple to do once you understand where to put the color and have the right tools.

First, use a very fine tip marker, something that is less than 1mm in size. Right now I’m experimenting with a “Penol 777 Fine Line Permanent” marker I bought at Staples. It works ok but if too much ink pools on the tip of the marker the panel line becomes too thick so I wouldn’t recommend it. The marker that I’ve used in the pictures below is a “Sharpie Fine Line” marker but I haven’t found any place to buy it in Sweden yet.

Second, hold the marker at a perpendicular angle to the surface so that you are adding a minimal amount of color to the body. The ink for the panel line should only go at the bottom of the seam so that the vertical surfaces of the seam and the body surface show the original color which then work to create the shadow effect by reflecting the light around but the bottom of the seam doesn’t reflect the light. That is why you have to use a very fine tip marker.

In this example cross section of a door seam on a blue body, the black area is where the marker should be used on the exterior and the blue would be the rest of the door seam which should not get any color on it.