You don't unless you have the 10.6 SDK installed and it somehow works with your version of Xcode.
Seems 10.6 SDK works up to XCode 4.3, and I can't install XCode 4.3 in the OS 10.8 which I was able to downgrade to (only Xcode 4.4+)
I just realised now that it seems I won't be able to install 10.6.8 because macs do not seem to be able to install any previous OS versions that they came out with. I'm not surprise to learn that, I'm just surprised I didn't know! I'm still trying to learn more about this. Can anyone confirm this?
The machine I was able to install 10.8 actually came out with 10.7, but I wasn't lucky installing 10.7 on it. And since I actually wanted to install 10.6.8 as my friend's computer, I guess I'll have to give it up.
The other way around I can think of is by getting a virtual machine running 10.6.8 - I've seen some torrent images for that.
cheers