1) Buy a Snow Leopard disc from Apple online
http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard - USA
http://store.apple.com/uk/product/MC573/mac-os-x-106-snow-leopard - UK
or telephone Apple. In the USA: (800) 676-2775 (contact information for other countries: http://support.apple.com/kb/HE57)
Alternatively you can buy from online retailers but anticipate paying much more than the original price which is the price still charged by Apple
2) Backing up your computer before upgrading is always a good idea.
4) After installing from the DVD, update your Snow Leopard installation using Software Update or manually downloading and installing the Mac OS X 10.6.8 Update Combo v.1.1 - http://support.apple.com/kb/DL1399
It is advisable to use Software Update at least once so you get special updates such as the "Mac App Store Update for OS X Snow Leopard" - https://support.apple.com/HT205702
If you are using old (PPC generation) software you may want to install Rosetta as well which I believe is an optional installation.
Read an earlier link which you may have missed during cross posting about upgrading to Lion.