1. Register for the co.cc affiliate program at http://www.co.cc/ or use my affiliate link ( http://www.co.cc/?id=167358 ) if you want to offer domains but don't want to register on co.cc's affiliate program, but it won't give you any money if you use my link. 
2. Pick either a link or a banner.
For a Link
3. Make a new EzBlock or board.
4. Post your link ( it is in this form if you have trouble finding it: http://www.co.cc/?id=167358 ) as the HTML code or the board description.
5. Done, you get 10 cents per free domain registered via your site, and 40% of the money from paid domains.
For a banner or iframe
1. Make an EzBlock or go to your headers or footers.
2. Paste the link in.

I run this on my web host, but it isn't popular so I only have 50 cents so far.

But warn your customers they won't be installable to an SMF For Free.