First things first, though. Before you needed a lot of fans to be able to set a custom URL for your fan page on Facebook, but in late 2009 they changed it to a minimum of 100 fans. Just before the end of the year people noticed that the minimum testing had fallen even lower, and as far as I know, only 25 fans to establish a personalized URL.
To begin, log into your Facebook, then in a new browser window or tab, go to the following URL:
http://www.facebook.com/username/
If you've already set a custom URL name for your own Facebook account, we will not like mine:

As you can see, the custom URL to get to my profile on Facebook is "facebook.com / naeem.lashari" ("naeem.lashari" was already taken by the time you tried to get, unfortunately). Below it is what we, however, "Establish a username Pages. The top three words are the link. Click 'em and ...

Check out this pop-up menu to see what pages you are eligible for a custom URL:

Now I can play and try different URLs shortened to find one that works:

That does not work because they need to have a minimum of five characters. Among one that works, however, and you get a nice pop:

If you like the name you specified and are cool with all the fine print shown here, then click "Confirm" and you just set up a custom URL for your fan page on Facebook, congratulations!