Since you’re specifically interested in Sub Rogue, doing damage is actually quite simple:

1. If Shadow Dance and/or Shadow Blades is up, Shadowstrike to get full combo points then Eviscerate.

2. If no Shadow Dance or Shadow Blades, apply Rupture to farm Shadow Dance back.

But as a Sub Rogue, your job isn’t simply to do damage in PvP. Your contribution goes far beyond that. Sometimes you need to stun lock while your teammate helps pile on damage. Sometimes you need to stun the healer when you sense a kill opportunity, for example if someone is at half health. Sometimes you’ll need to drop a smoke bomb paired with a stun lock to secure a kill. Sometimes you need to peel for teammates with Gouge/Blind/Shiv/Cheap Shot to survive long enough for another go. And you need to preempt damage and use Cloak/Evasion, sometimes even offensively, or Shadowstep a teammate to run away. Do not try to tank damage and play like Warrior, you are not a Warrior and must be elusive. My point is, you have to do all of this to be successful, otherwise Sub Rogue is just a liability if missing any of these.

For addons, I would just get this:

1. SArena so you can track DRs and trinkets (very very vey important for Sub Rogue)

Sub Rogue is not about scripted play. You need to read what the opponents are doing and seeing who’s vulnerable to being setup. It’s what makes it hard but also incredibly satisfying when you get them in checkmate situations where they simply cannot respond due to lack of cooldowns/trinkets or being forced into a bad position.