Hi Gil,
http://msdn.microsoft.com/en-us/library/c76dd5k1.aspx
should help you, you should not use "portal" as a prefix for your custom controls though, you should make up your own prefix.
Note that while the article shows how to register controls in a page or other control, it can also be done from Web.config, if you look in the <pages><controls> section of Web.config you will see where I have registered the built in mojoPortal controls using the portal prefix.
Hope it helps,
Joe