Installing Windows via .iso

So I have a (base model) rMBP which of course means no optical drive. I downloaded an OEM version of Windows 7 64 bit to see if I could get it to install, and if so purchase a license for it.

So I tried installing via popping the .iso on my external Samsung hard drive, and then I tried using Daemon Tools Lite. Both methods resulted in 'No bootable device' (I had a slight panic attack until I realised via my old Dell that this didn't mean I had broken my Mac!)

Anyhow, does anyone know how to get this to work? I've had a Google but almost all forum topics I find are to do with installing via optical drive, and I really don't wanna buy one because I will never use it again.

I realise I am probably doing something really stupid but since this is my first Mac I rescind all responsibility

 

Ok, so an external hard drive won't work? What size flash drive should I get?

 

I must have done something wrong the first time because it installed this time without a hitch from my external hard drive. But geez, does it run hot!