Your job is to be between the enemy team and your backline. You are a wall. Walls don't chase kills. Walls don't flank. Walls stand still and absorb pressure.
Positioning: in front of your ADC and mage, within dash range of the enemy frontline. Your goal is not to kill anyone — it's to make sure the enemy can't kill your carries.
When to engage: when the enemy carry mispositions. Not when you feel like it. Not when your team is pinging you. When you see a clear opening.
When to peel: always by default. Only switch to engage when you see the opening.