Two ways...
An Existing iFrame App - Requires nothing but FB Page
1.) Visit
http://www.facebook.com/search.php?q=iFrame&type=apps&init=srp
2.) Along the left side of the page, look for "Add to Page"
3.) Click "Add to Page" and select the page you want to add it to
4.) Visit your page, you will now see a "Welcome" tab
5.) Click that tab, and edit the source code for the tab
6.) Save settings
7.) Go to your page settings, then "Manage Permissions" area
8.) Set "Default Landing Tab" to your new tab
9.) To edit the name of the tab, go to "Apps" within your page settings, then click "Edit Settings" under the Static HTML app
Create an App - Requires site, application, FB Page
1.) Go to developers.facebook.com and create an app
2.) Select "Page Tab" within app settings area and edit the details, Tab Name, Page Tab URL, etc.
3.) Configure the rest of the app, such as your base domain, the canvas URL, the icon, the picture, etc.
4.) Save the settings, and visit your new app page (facebook.com/apps/application.php?id=yourappid)
5.) Along the bottom left side of app page, click "Add to Page"
6.) Select the page to add it to...
7.) Go to your page
8.) If tab isn't visible goto page settings > Apps > "Edit Settings" for your app, and click "Add"
9.) Set the custom tab name too, if you'd like
10.) Create an index file with your source to be displayed when someone clicks the tab. This should be placed in the directory you set in the application under "Page Tab URL"
11.) Go to your page settings, Manage Permissions, and set "Defaulting Landing Tab" to your newly created app