Clean install of Yosemite on 2008 MacBook Air I am trying to upgrade a 2008 MacBook Air from OS X 10.5.8 (Leopard) to 10.10 (Yosemite). I understand from the Apple web site that the minimum required version of OS X is 10.6.x (Snow Leopard) for an in-place upgrade. However, I am assuming —correct me if I'm wrong— that the pre-existing version requirement isn't relevant if doing a clean install from a USB boot disk or other media? In any case I have had trouble doing this, hence this post:

I have found several guides on how to create a bootable USB stick for Yosemite, and my colleague has kindly created one for me using her newer MacBook. The guide used is located at http://www.guidingtech.com/34311/clean-install-yosemite-os-x/. I have attempted the install using the stick, by pressing the alt key after power on and selecting the USB stick but I am then faced with a No entry sign and the Air shuts itself down.

I would like to know what the most appropriate approach would be to get Yosemite working (legally). I don't have a Snow Leopard disc, and ideally I'd like to do a clean install anyway.
What you're encountering is not related to the pre-existing version, but likely that your MacBook Air is not supported for Yosemite at all.  Apple lists the Late 2008 model as the earliest MacBook Air which can run Yosemite.

(Source) http://support.apple.com/en-us/HT6412