How to remove the controller/method for cleaner URL in codeIgniter.

The original url is below
http://www.mydomain.com/controllers/method/variable
and I want my url look like this
http://www.mydomain.com/variable
or http://www.mydomain.com/friendly-url-description/5
5 -> is key or id of the table, and the friendly-url-description is description of the value. please help guys.