It's gonna be hard, but since everyone is asking about it, I think I will give a try
Get the TWRP official app and TWRP manager, one from TWRP and one from jmz software
Install both softwares and make sure you can get the TWRP app working (This is very important, because the reason we need a computer is because TWRP often requires a computer)
In the TWRP app, flash SuperSU. You can just slide it in
Possible issue: It is sometimes difficult to get the TWRP app working without a comptuer. I REALLY RECOMMAND GO TO A INTERNET CAFE AND PAY 30 minutes of INTERNET to manually get your SuperSU installed. YES my first root is done in the cafe, with a very crappy computer. You don't need a good computer to do it, just A COMPUTER is fine.