how to really permanently delete everything on samsung s4 active? I will give my phone to technical service due to a problem on battery. I am a little paranoid about technical service employee and don't trust them in terms of personal privacy.

I read that factory reset does not permanently remove everything on phone:
http://blog.avast.com/2014/07/09/android-foreniscs-pt-2-how-we-recovered-erased-data/.

I would like to delete everything on phone in a way that in which even high level forensic recovery softwares can't recover anything.

By the way, i am not an expert user on android so please describe the solution in detailed form.
Here are the steps - 

1) Power off your device completely.

2) Press and hold Volume Up, Home and Power buttons simultaneously.

3) Select wipe data/factory reset and Wipe cache

Here's a Video for further help - www.youtube.com/watch?v=vNfRb13i7vI