First step, and this sounds like a no-brainer, but sharpen your white pencil. Second, stamp your image with your favorite white ink. If it is a pigment ink, you may want to let it dry overnight or heat set it.
Once your ink is dry, take your pencil and start going along the lines of the stamped image. If it's curved, curve with it; if it's straight, follow along that. This cupcake is a great combination of curves and straight:
Notice how I didn't make my colored lines solid in all areas. I like to leave some black showing along the stamped image to make sure I don't lose it with all the white. Next, go back over some of your lines that you would like to be more white. Lay down your first line of color slightly within that first colored region.
See how you're starting to see some dimension forming? The great thing about this technique is you don't need to be pushing hard on your pencil, you can layer your strokes to make it a stronger line. Just put another layer on until you like the way that it looks.
Here's my finished image. The hardest thing about this technique for me is deciding when to stop! You'll see what I mean in a minute.
So here it is with the Robe Image. It's the same process here, first, stay pretty close to the lines.
Second, add in a little more:
And third, figure out where to stop! Here, I went a little too far (not too happy with it, but wanted to show you what I meant about knowing when to stop, lol).
So now you know just how easy this really is AND you should try it! I would love to see your results, so if you get a chance, email or leave a comment with your link!