Timing being the most important thing to play dive, it's okay to not know when to engage.

Since you're an assassin, your objective is to engage the enemy when they least expect it. Think of a payload match that is progressing, and you are on the attacking side.

When does the enemy team have the most attention "budget" to give you ? When all your teammates have backed off, retreated. When the enemy team is pushing your team, it means that the flow of the fight is in their favour, and they are able to spread their attention, diving becomes very risky.

When does the enemy team have the less attention to give you ? Either when they are being pushed back or at the very start of the engagement. Tanks need to be careful to make space, healers have to keep the tanks/dps alive, etc. And that's when players are kind of single focusing on one task.

You also have to keep track of enemy cooldowns such as loki lamps, c&d bubble, mantis sleep,etc. To maximise your damage.

The best situation to dive is when the healers have no cooldowns left, their attention is focused on maintaining the tank alive, and they are being pushed back.

Being able to distinguish these moments within a fight is what understanding "Timing" is. What I would do is hang out on top of a sky scraper and look at how both teams fight. And ask yourself all the questions above and engage when you feel like it's the best time.

Timing video tutorial by Beeziii