You will need: A second Mac (with FW or TB port) running Snow Leopard, a Firewire or Thunderbolt cable, a Snow Leopard installer (DVD or image on the second Mac).
1. Bakup all your stuff, it’ll be gone! 2. Connect your Mac to the second Mac which is running Snow Leopard via Firewire or Thunderbolt. 3. Boot the Mac you want to downgrade into Target mode (press and hold T after startup chime). 4. Once the Mac shows up as an external hard drive in the Finder of the second Mac, delete/reformat the drive with Disk Utility. 5. Now start the Snow Leopard installer on the second Mac (DVD or image). Choose the external hard drive you just erased as the target and install Snow Leopard. Quite a bit of work but it does the trick. Thanks Apple for making it so easy, not!