The easiest route would be to just use a program that helps you automate things, like the above mentioned Hazel. Or a free alternative:
Next on the ease of use ladder would be to take the Applescript snippet above and build it into an automator action.
You could save the snippet as a script in script editor and add it to the OF toolbar (see the extras forum for more info on how that can be done)
Finally the Unix command can be added to a file that the computer will run every night at midnight. located in /etc/daily.local
None of the above options are explained enough to follow as a step by step set of instructions, but depending on how much you want to learn, you can pick from top to bottom ;)
Good luck - there are lots of resources out there for learning about Applescript and shell scripting.