Backing up question in GHOST 14

I bought Ghost because I want to do the following things:

 

-Backup certain files onto my external USB hard drive.

 

and 

 

-Setup Ghost once a week to run this backup again BUT only backup files that have been CHANGED or CREATED since the last backup.

 

How can I do this?

I bought Ghost because I want to do the following things:

 

-Backup certain files onto my external USB hard drive.

 

and 

 

-Setup Ghost once a week to run this backup again BUT only backup files that have been CHANGED or CREATED since the last backup.

 

How can I do this?