downgrade from os x capitan 10.11.1 to os x snow leopard 10.6.8

Do a backup, preferably 2 separate ones on 2 separate drives.

OS X El Capitan: Revert to a previous OS X version

If you do revert, I'd use Setup Assistant to restore your data. This process takes a while, so do it when you won't need the computer for several hours, based on my experience.

Erase the hard drive and install Snow leopard from the DVD.