There are several tutorials available, but many of them show steps for an older version of Facebook *) . Here is what works as of December 2011:
- Facebook allows you to create a page (you can call it a "tab"), and place a link to it at the menu on the left.
- That page is able to show an external content, "embedded" to it via <iframe>
- The "page", in fact is a Facebook Application, with a very little capabilities. To make a simple page, nothing is required but making an application and specifying which external URL it will show. The rest is done externally, on your own webserver.
- Your external URL should be available via https: (SSL). That's a new requirement of Facebook, probably because Facebook itself jumps to https automatically (if Account Settings > Account Security > Secure Browsing (https) is "on"), so all embedded content should be secure, too.
- There is a weird way to link your new application to a page:
The "next" parameter can be different, but for a simple page that you make for yourself, using the above "dialog" URL is enough.
- You should setup your external page style using Facebook CSS, so it will look like a part of Facebook. I think that having the same font family and size is very important for a simple fan page.
- There is a way to get information about your page visitor using Facebook API, but again, for a very simple Like/DoNotLike it is not required. In my version, I do not even validate the request, so hackers are welcome to reveal the coupon code by crafting a Facebook request (takes more time than pressing the [Like] button though :-)
*) Tim Ware's HyperArts Blog has the most up-to-date information