Help rooting GS6(SM-920i)

I recently purchased a galaxy S6 (SM-920i) running Android 5.1.1 in the hope of rooting it and flashing it with a custom rom. I'm a novice at this, and have not been able to find a decent tutorial on this subject. I'm not concerned with tripping the knox counter by the way. Any help in this matter will be much appreciated. Thanks.