How do I install Snow Leopard on a external hard drive?

I am repairing an old white intel iMac and I want to replace the faulty hard drive with a new hard drive with Snow Leopard OS via my 24" iMac running Yosemite. Yosemite will not let me install Snow Leopard on my external hard drive. I end up with the Snow Leopard install showing a large O with a cross through it and that is were it ends for me.

I have looked at the various You Tube tutorials but none address this issue of installing an older OS on what is for me an external hard drive to be eventually used as the hard drive for the old white intel iMac.

Suggestions gratefully accepted.

Thanks

via ehMac.ca http://ift.tt/1CzABDR