Download the Tor Browser Bundle for your Operating System (windows/Linux or Mac) from http://adfoc.us/3119924296500
Install and run it.
You should have the Vidalia Control Panel somewhere on your desktop.
Click on Setup Relaying.
You have a few options here:-
Non-exit relay (just pass traffic around TOR network)
Exit relay (pass traffic out onto the internet on behalf of other users)
Help censored users reach the Tor network (makes you a hidden connection that people in countries that block Tor can use to get to Tor)
I chose ‘Relay traffic for the Tor network (exit relay)’.
I also chose >1.5Mbps bandwidth.
Once done here, go to Exit Policies.
Tick all the traffic you are happy to allow out through your internet connection for other people.
I ticked all of them except for Misc Other Services but this is a personal choice - as some may allow abusive traffic.
I then clicked on basic Settings.
Ticked the box ‘Attempt to automatically configure port forwarding’.
Clicked the Test button, this showed everything as Ok.
I clicked on close on the Test popup window.
Click on Ok.
I did this on Linux, hopefully the instructions are the same/similar for Windows and Mac.