Hi,
I did the same thing yesterday (instead of an external, i just partition a piece of my HD to run ML in. Installed and expanded, ML is about 10 - 11 GB. I partitioned 40GB so I can have some room to play and test).
But all you have to do is once you have your partition made (or external loaded onto your mac). Run the installer, it will ask where you want to install it. (just don't point to your main HD).
After the install, you can reboot your computer, and hold the option key and you will see your various OS instances and you choose the one you want.
Simple actually.