Downgrade from SlimBean 4.2.2 To Stock Rom 2.3.3 , 2.3.4 or 2.3.6 topic

Greetings,
I Have Galaxy S I9000 and I'm on Slim Bean 4.2.2 i want full erase my data and downgrade to stock ROM 2.3.3 , 2.3.4 or 2.3.6 cos i will sell phone and i don't want the buyer reach to my old stuff and he need the software on stock could anyone help me with the instruction and steps provided by links plz .