Automatic switching between roles

Hello. Can someone help me out here. I found this topic thread and it has a solution to my problem. The issue now is that I don’t know where and how to write the solution code that was given in the thread.

http://wordpress.org/support/topic/auto-change-user-roles?replies=7

I want to have user roles changing from one role to another automatically after X amount of days from the time I give them a particular role. For example I make a subscriber an author today, then 30 days later their role must revert to subscriber. If I change again to author after 5 days, then 30 days later the role must switch back to the subscriber role. I therefore need to know where the given code will be put and where I will define the period (X days). I am not a programmer but if someone gives me clear layman’s instructions I will be able to do it. Thanks in advance.