Hello and welcome to my guide, it's my very first guide and ill try to make it as simple as possible for you start playing Zac with everything you need to know for your very first time or to clear off some confusion.
I started playing League in 2023 and the only thing i liked was Zac, so you can say i OTP Zac, but i had my support phase/other jg champs.
I reached Master playing Zac back when Mythics were a thing and i miss some old itmes, they were so good for Zac but they removed every one of them :(
I wanna start steaming my journey to master and maybe some other games, so if you're interested and wanna ask me things about Zac make sure to check the socials tab, i'd appreciate it!

I always like to start with the red buff-krugs-raptors and so on but you should alaways look for your bot lane's comp and if you have an easy gank just start with blue-gromp-wolves.
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In case they steal your other part of the jungle assuming your team watches for you, just take the enemy jungle and gank lane right away for the best value OR you don't know your jungle was stolen and you just noticed just try to look for a gank or a scuttle fight with your lanes etc. Don't let him take your jungle for free, you're better.
The scuttle fight is always bad for you, especially that Zac is getting beaten by most of the enemies early game, so either try to call your lanes for help, go for it before your enemy jg comes or just take it if the enemy is on the other side but make sure to ping your lanes that the enemy is there

For mid game you're just seeking ganks, your E is long enough even from level 7 to make your job easier so try to gank as much as possbile and make sure your jungle is warded or you might get taxed for your actions. Make sure to take objectives but you already know that part <3
For late game and team fights, try to jump in the backline, focus on the carry and CC him as much as possbile. You're the engager, play around vision and also with your team, playing solo if you're not ahead is just suicide.
Taking risks to jump on Vayne, Varus, anything that stuns, cancels your jump, stun you is not worth if you're not sure you catch them. Be patient!

There isn't much you can do with Zac, most of your fights are just landing your Q on 2 enemies, E on the center of them and then R afterwards with a lot of W spamming and collecting your little chunks.
Combat wise, you can also Q + flash enemies for a better and unexpected move
BUT
You can E over drake, smite and steal while mid air( You need to be a little bit precise with it) also you can avoid vision, place vision while in E, Q one enemy E on the other one and Q him to clap them both.

There isn't much to say about items if you follow the sets above but i have some things to clarify.
First of all, i don't like Unending Despair even tho everyone says it's a good item for Zac, i feel like there are a lot of other items that gain more value than this. You can test it come back with a feedback but i've had my experience with it and i don't like it.
I feel like you should consider buying Frozen Heart if you don't have a support that does so when needed, also Bandlepipes is a wise choice if you have good m1 champion in your team.
About Mejai's Soulstealer i think you should stay on Dark Seal if your team is not ahead and you have 10 stacks on it. It's just a gamble to buy Mejai's instead of getting solid items. Get it only if you're over 100% sure you will get value out of it and not just go for "I might kill them if..."
Buy Protoplasm Harness if they have a lot of burst damage, if not this item should not be considered

My future streaming platform: https://kick.com/androniestefan
My Discord if you have any questions: stean.69
My LeagueOfGraphs: https://www.leagueofgraphs.com/summoner/eune/Marlboro%20Red-6767

If you have any more questions or not sure about something you're more than welcome to message me or come by if i ever stream.
Much love to all of the Zac players and good luck on your journey!