How to back up computer properly in order to restore to factory settings

Hi! I have a late 2008 MacBook Pro that I think was originally running on Leopard. I had my friend upgrade it to Lion about a year ago (it says it's running on 10.7.5), and it's been running horribly. I want to restore it back to Leopard, so I backed up my computer to a WD Elements portable hard drive. However, I read on one of the forums that a back up won't transfer over to Leopard properly because everything is in Lion format. It also mentioend something about a carbon copy, but I wasn't sure if that was an actual step or just a description.

What steps do I need to take to properly back up my computer properly so that when I restore it to Leopard, everything can be easily added back to the computer? And how do I restore the computer? Would it be better to ask someone at the Apple store to do it or can I do it on my own? I don't think I have any discs that came with the computer (if any did).

Thanks so much for anyone's help! I was getting really confused by all the different discussions regarding this issue.