Question Galaxy S2 HD LTE (SGH-I757M) help

Hello I need help ..
I have my phone sgh-i757m with android 4.1.2 root I made the stock room.
now I want to sell the phone and remove the root as I sometimes stays frozen on power.
I have the android Reduced sammobile.com page.
the question is what are the steps to follow tutorial pls if possible.
some link.
sorry if the English is not perfect and I did it with google translator