its really easy. I do it all the time when I need internet in a pinch.
1) A Jailbroken iPhone w/ PDAnet (you can download it from Cydia)
Turn your Airports WiFi signal on in OS X, and select "Create Network..."
A new prompt comes up asking you to name your new wireless network. I usually pick something like "iPhone-Tether-3G" or something like that. Then left the channel to 11. Check "Require Password". (you dont HAVE to, but this would stop those others in your vicinity to connecting to your open WiFi network.)
I usually only pick a 40-bit WEP password mainly because I only use the 3G internet signal for a very short time anyway. Type and retype your password then click "OK".
Now --- go to your iPhone and tap "Settings" >> "WiFi" >> "ON"
Select the network you just created, and type in the password if you had one. It will connect at this moment, it might look like your still in "3G" mode since the icon might not change but not to worry keep going.
Then, go back to the Home Screen and tap "PDAnet" >> "Settings" >> "WiFi Router Mode" >> "Done".
PDAnet will then connect and you should be all set! Go to your Mini 9 and open up Safari and it will take you to your homepage.
Some quick notes you need to know. If you do not buy PDAnet then the trial version will only allow you to surf webpages, you wont be able to use other ports for things like AIM, Skype, etc.
Still, its better than nothing and if you like how easy it is, buy it. Im sure you'll be using it a lot. Just be careful as AT&T hasnt fully announced yet that you can use your iPhone for tethering purposes. So only use this for when you really need it.