Jeff is unique in that his playstyle HEAVILY changes based on the team composition. As such, we will discuss how to play Jeff in each of the major 3 team comps.
In dive, Jeff can fulfill 2 seperate roles. One, he can join the divers in their dives, hiding in flank positions or off angles. Jeff's m2 is wonderful at comboing and catching people off guard, allowing them to be easily picked off using his m1 or by an ally. Jeff can additionally solo dive support's, forcing the supports to change their attention to him instead of healing their team. Even if Jeff doesn't secure a kill, this can be extremely benefecial to his team. Aside from pressure, Jeff is still a healer; he can heal his team from a very long distance using his m1, or apply bubbles to his team before they dive in. Either way, they will be healed mid dive, prolonging the fight and increase the odds of securing a kill. Jeff's ult can be used more as an expensive 'get out of jail free' card, as it allows him to be invincible. Otherwise, the general rules stay the same: apply bubbles during fights, survive as much as possible, and get kills.
Jeff is mainly relagated to a healbot in brawl, sitting in the backline and applying bubbles while holding down m1. Depending on if the team is running 3 supports or just 2, Jeff can shift to a more offensive role. If they run 3, Jeff can dive alone and distract/kill the enemy supports. If there are only 2 supports, Jeff must play a more heal focused role. With the upcoming change to Jeff's ultimate in season 4, Jeff can use it more as a typical support ultimate, allowing his team to survive for longer. Of course, aim to get as many supports in your ultimate as possible, ensuring your team isn't at a major disadvantage.
Arguably Jeff's worst team composition. Jeff will need to position himself very well to ensure decent healing, as most poke comps tend to not be as together as a brawl comp. Jeff will need to land his ultimate much more carefully, as his team cannot follow him once he iniates it. As such, Jeff should ONLY ult when either the entire team is vulnerable (or at least half), or an enemy support is ult'ing (using their ultimate). Jeff will struggle to work, but he can still fit, provided you play him carefully.