Put your phone into
Download mode by turning it off completely and then press and hold the following buttons...
Volume Down +
Home (centre hard key) + (lastly)
Power. Continue to hold these buttons until your phone boots into the
Warning! screen asking you to press volume up if you want to continue or volume down to cancel. Press
Volume Up. See video,
here.
If you need to return it to stock you will need to download the stock Samsung firmware for your model of phone and CSC code and flash it via Odin. See
#1.7 of the
Jelly Bean update - Dummies Guide for full instructions, downloads and video on how to do this.