Boot the phone into recovery mode using Volume up button + Power button
In TWRP, if you get a prompt asking you to swipe to allow TWRP modify the system partition then go ahead and swipe. To swipe, place your finger on the green circle then slide it to the right
In TWRP recovery, tap Install
Navigate to the location of the SuperSU.zip and select it
Swipe to confirm flashing of SuperSU.zip and wait till flashing is complete