1. To begin using this technique on an ATC, you need to use a 2.5x3.5 piece of background cardstock. Or, a little smaller if you plan to mount on additional cardstock, as I did. Stamp your image in the center of the cardstock piece. Mine happens to have a rectangular outline around it, which is helpful, but not necessary. The technique is called "mono" masking, part of this meaning your going to be working in one color group, so make sure your ink and cardstock are from the same color family.
2. Next, cut the sticky end of a post-it note to the approximate size of your central image and cover it. Again, my image had the border around it, so it was really easy to cut to the right size, but don't worry if you don't have a border around your image - it will be added later. Fold the post-it in half lengthwise, unfold it, and then fold it cross-wise and unfold it again. Then stick it down over your stamped image.
3. The folded lines will be your guides for the next steps. Take two new post-it notes and line them up in the center boxes so that they cover two corners of the cardstock and leave two corners exposed. Stamp a background stamp or a small stamp repeatedly to cover the open areas of cardstock.
4. Pick up the two top post-its and move them to the opposite corners, so the corners you just stamped are covered and the unstamped corners are exposed. Stamp a different background on the exposed corners.
5. When you remove the two top post-its, you should see all the background now. Use a stipple brush loaded with ink to lightly colored the exposed background, coloring a little heavier around your center post-it note.
6. Now, using the lines on your center post-it as a guide, use a marker in your color family to draw lines around the center image and in between the backgrounds where they meet. When you remove the center post-it, you'll see the stippling makes your border a little darker and it looks like a layered piece, even though you're working only on a single sheet of cardstock.
7. Finish your piece as desired. I did add a little pop of color to mine, because it was very brown, but I've seen the technique in blues and other colors where you don't need it.
Hope this helps you guys - enjoy your mono masking!!