HOW TO CREATE A CHILD THEME STEP BY STEP GUIDE?

HOW TO CREATE A CHILD THEME STEP BY STEP GUIDE?
Please i need a help, do i really need to create a child theme for those WPMU DEV themes? or not?

Please advise how to create a step by step procedure on how to create a child theme, especially if there's a video tutorial on this on how to create a child theme??

Thanks a lot. More power to wpmu dev!