I'll first introduce the technique with a simple logo and then move on to something bit more flashy.
I have to apologize about the watermarks on the images. The article was originally published on my modding site: http://metku.net/
Step 1: Idea
To show the basic princible I selected the Intel Inside logo. It has three distinctive objects that can be divided to separate layers.
Each layer is engraved by using whatever engraving tool you like. I prefer the Dremel Engraver for more detailed work and regular Multitool for larger surface areas.
Impotant note: As we are lighting up one layer at the time, one should try to keep the engravings from overlapping one another if possible. This is not that much of a problem if the image is a simple line drawing but if the topic calls for larger sanded/engraved areas, they will block the light coming from the layers behind them. Just something to keep in mind when selecting the ideas for the image.