How do I create a boot USB from an older version of OSX?

A friend of mine bought an iMac (2008) but apparently it wasn't wiped properly. After much messing about and failures to install OSX, I think it is because I am downloading the latest version. Unfortunately I can't download ElCapitan because the version on my Mac is newer. I have checked my purchase history and I haven't ever downloaded OSX so can't go that route. The other option that I have read is to get DVD's of SnowLeopard and upgrade it from there. There has to be another way? I have searched through so many forums and can't find anything that helps me.

I don't profess to know much about technicalities on Macs. They work so that is all I have been interested in after many frustrated years of using Windows, so please talk me through in baby steps if you can.

If there is anyone out there who can help, it would be much appreciated.