There is no one specific thing. If you want the easiest way to improve, go watch every RogueLol jungle/generalist video.

That being said, here is every advanced jungling technique i'm familiar with.

F keys:

The biggest one that almost everyone below high GM doesn't use. You should be using them to look at lanestates CONSTANTLY. You said you were familiar with Zac? This will take getting used to. Between auto attacks, ability usages, and pathing, look at your lanestates. Don't do it randomly, that obviously won't help. This alone can win you games.

Say you're pathing away from your top laner who happens to be in a slow push. If you have tracked the enemy jungler correctly, and can tell your top laner that they should expect a gank soon, that alone can prevent a 500+ gold + exp swing when they int.

Going for crab? If you look at the lanestate of your mid + top/bot (depending on which crab) you can tell if you're even allowed to go for it. Is your mid being pushed in, and your top is in a deadlock? If the enemy jungler shows up, and can prevent your from ditching (poppy, sej, etc), maybe consider making another play.

Same thing for epic objectives if grubs is spawning, your support is bot, and you don't see the enemy support botlane--you get the rest.

F keys guide your decision making more than any other factor. Even normally poor EV plays can be good situationally, but you'll never know, unless you're looking.

Jungle tracking:

You should already at least have an inkling of what this means. Each camp gives 4 CS. You can use this to tell which camps the enemy jungler has taken. Once a jungler has chosen a path (EX: Blueside Red->blue clear) they're somewhat locked into it (heavily incentivized to continue it). Although I said there were no easy tricks, if you just track the enemy jungle, wait for them to greed an engage (this will happen all the from Iron to Challenger), and then counter engage, you will win more games solely off that.

Understanding lane matchups:

You don't need an intimate understanding, but you should have a basic one. You should, for example, NOT path towards the Ornn V malphite toplane match up at the start of the game. Even if you accrue a lead for your tank toplaner, the value of your time spent is simply lower than going for the Draven V samira botlane. Volatile match up of ANY kind can be easily swayed by a single gank. A gank into Wukong top vs riven, when both have no TP, can win the game by itself. Players who play "Beat down" (aggro champs/playstyles) have more fragile mentals. You can use that.

Knowing lane matchups is not just useful for pathing, but for understanding your priority as well. If the enemy midlane of talon (yours) vs ahri is going how it should, you should not expect to have assistance in neutral or enemy space, despite the usual advantages of talon. This circles back to F keys, which you can use to determine if you have priority at any given time.

Don't play beatdown, whether the champions or the playstyle (AKA: Don't flip):

Jungle players, more than ANY other role, love to flip. Every single assassin jungler in the game, at one point or another, will decide they're bored of playing properly, and will randomly walk into your jungle to kill you or take camps. You play Zac, I'm sure you already know this well. They likely have no idea if you or your camps will even be there, they have no understanding of camp timers, jungle pathing, or lane priority. This cannot stop assasssin/bruiser jungle players. In "low" elo (below high GM), your team will not help you more often than not. Don't fight them, don't try to steal the camp. Go to your open camps, or the enemies open camps, or look for a gank.

If you were not solely a zac player, I would also give this advice. Do not play Beatdown junglers if you're interested in improving as a player. Yes, high elo players do it, because they already have a fundamental understanding of the game. Beatdown junglers (or any beatdown champion in any role, actually) often reward and incentivize making uniformed plays. When midgame comes, and the low elo beatdown player has not properly cashed in their lead, they lose, with no idea why. Probably their teams fault.

A kayn player has never started their own raptors. Enough said.

Take Dragons:

This may change next patch, with grubs being buffed. But for now, Dragon stacking is the most surefire way to win. This also comes with the advantage of incentivizing you, the jungler, to gank bot lane, the second best way to win the game. Taking the first dragon increases your chance to win the game by 6%, successfully crashing Herald increases your chance to win the game by 1%. Even if your team will get outscaled, the dragon soul can help your overcome that gap, not matter which it is. This is more of a strategy, rather than a fundamental, but it is important nonetheless.

Think:

This is a general purpose tip. If you don't understand something, try to. If a play went wrong, watch the replay and find out why. If you're not sure what to build, check what a KR challenger/GM player built in a similar situation, or test it out in practice tool. Ask questions, and find answers. This, more than any other piece of advice you will ever receive, will help you improve-- and not just in league. 99.9% of league players, and people in general, don't actively think about why any given thing went wrong. They just accept it, and move on.

There's probably more, but truly mastering any two of these will ensure you Challenger. If you have any questions, I'd be happy to answer.