handling backups

I wish to set up the grandfather/father/son approach whereby I create daily differentials and weekly full backups. The dailies are to be kept for up to 6 days and then the disk space is made available for reuse. Likewise, the weeklies are to be kept for up to 4 weeks, and then serially freed up and/or disk space made available for reuse.

How do I set that up? I can find no retention policy for the *.tib files Acronis creates.

Thanks.