To access the tool, go to Google Webmaster Tools, then click on tools on the left and then click on “Enhance 404 pages.” You can then copy and paste the widget code from the Google site into your page template. For example, I set up a page at the Search Engine Roundtable at seroundtable.com/advertize.html; notice I spelled advertise with a “Z.” Google automatically notices that I have a page at advertise.html, and offers that as an option for people to visit. Here is a screen capture:
You can customize the page with your template or other design. For more information on how to do that, click here.
Postscript: It seems like this only renders in Safari; maybe it was something I did, but all I did was copy and paste the code into a blank page. Tamar posted more details at the Search Engine Roundtable, and thanks to our readers for the tip.