This is a fun technique that has been around for a long time. Lots of different looks depending on which stamps and colors you choose to use. I've seen it done two ways.
One method is to choose a solid/bold image stamp and a background stamp. Ink the background stamp and "kiss" it to the solid/bold image stamp. In other words, stamp the inked background stamp on to the solid/bold image stamp.
Then stamps the solid/bold stamp on to paper. You'll have the shape of the solid/bold image stamp with the detail of your background.
The other method I've seen is to ink the solid/bold image stamp with a light color ink, then ink the background stamp in a darker ink and "kiss" the solid stamp. If needed "huff" or breath on to the stamp to keep the ink moist and stamp on to paper.
With this method the bold image is prominent with the detail added. Both are cool.
Depending on how you want your image to look, use a much darker ink for the background image as seen on the right below.
If you choose a less detailed background stamp, the basic bold image will not be as clear if you only ink the background stamp (as on the left) and not both stamps (as on the right). Both are OK, it depends on what effect you are going for.
You don't have to use background stamps. You can stamp other images on to you bold stamp. Here is a butterfly on the left and a fairy on the right. Here it is needed to stamp the bold stamp, or you won't see the outline at all (when using images smaller than the overall size of the bold stamp).
Here's another sample. Remember that the image you stamp on to the bold stamp will be a "mirror" image of your original stamp. This means words and numbers will be backwards, unless you have those reverse image stamps Tim Holtz makes (and maybe others are out there also).