Hello,
You will need form BI-1664 & BI529 if you type in google these forms you can find them.
Copy of your green card.
Letter stating why you want to retain South African Citizenship.
Copy of your South African Passport.
$25 fee Money Order
You need to do this before you become a US citizen. As far as getting the documents notarized you can just go to a UPS store and ask for a notary that is what we did.
You can PM me if you have any other questions. We did this a couple years ago.