Well, little new player of Jhin, he's a particular ADC, similar to noone else in the league. ("What about Graves?" We don't talk about the incident, Jimmy.)

His Weapon, Whisper, is a reload-needed gun: you have four ammo and after the fourth you reload. His Attack Speed cannot grow except by level and the reload animation last 2.5 seconds, no matter what you do.
BUT every attack speed and crit chance percentage you build becomes another % that will scale directly on your total AD. Also, you gain another percentage of AD from just leveling up.
Your crits are... not the best, making exception for the fourth that is ALWAYS a crit, and not a simple one: you will deal 15% to 25% of your target's missing HP as bonus physical damage.
As i said, your critical AAs are not strong as crit theirself BUT consider this: through late game you're gonna get around 600 AD, and if you do buy the Infinity Edge your crits will deal 187.5% of your total AD. Not that bad, ay?

One last thing: i'm not going to explain every skills of him. For that, take a look to the wikia http://leagueoflegends.wikia.com/wiki/Jhin The reason is simple: i don't like wasting time, mine or yours, AND you will understand how Jhin really works playing him. Seeing the numbers will not help you a lot, trust me. They like to change too frequently.

So, why Dark Harvest?
Why not that other runes like meteors from the sky, a magical kitten that throw itself to an enemy, a weird foot that will make you run etc etc...
Because they... sucks a bit on Jhin. Tbh Arcane Comet is not that bad on him, in fact i shall say before trying this build, try one with the comet instead, than this. I'm not selling anything, you need to find the perfect one for your playstyle.

Now, where were we? Domination's Dark Harvest
This rune is FOR ME the perfect one for Jhin: it gives you on-hit bonus damage for 20", it stacks up with Duskblade of Draktharr's AA proc and after picking up 150 soul essence it has 5 minutes cooldown before it goes away. You only need to pick a soul on the ground while fighting and trust me, IT'S NOT HARD: Dark Harvest takes soul from both enemy and ally champions, from big minions and from jungle monsters, it's not hard at all. Every time someone die you get a free soul that transforms in free bonus damages. Just... don't wait your allies to die.
Electrocute is "k", not that strong on a literal oneshot champion, and after killing one enemy champ you're like "k, my job is done...". You're gonna deal damage anyway, you're freaking Jhin. But... "k".
Predator is just... no. Tbh is not that bad but you don't run like Hecarim or Quinn so no. Just... don't.

Cheap Shot or Taste of Blood? Is up to you to decide. Cheap shot deal some true damage to impaired enemy champions on a 4 second cooldown, pretty ok, Taste of blood gives you heal upon damaging a champion on a 20 seconds cooldown. I play almost forgetting bout this two in game, not because they are not "game-changer", only because you're gonna deal tons of damage anyway, and heals a lot anyway, so yea. Meh. I am not explaing why Sudden Impact is not a good rune on Jhin.

Eyeball Collection 'cause more AD. The other two runes on this subpath are not that worth, you WANT that bonus AD from their eyes.

For the Hunters subpath choose yourself: if you know you want more spell vamp (you deal tons of damages from your abilities too) take Ravenous Hunter;
want some cooldown on your Mercurial Scimitar, Youmuu's Ghostblade and Farsight Alteration ward? Take Ingenious Hunter;
want to run, run faster with Relentless Hunter
From my POV: always Ravenous Hunter.

The second path is up to you, again. But i prefer the combo Domination + Precision, with Triumph and Coup de Grace for extra heal and damage.

This build, probably you already guessed, is an "on-hit proc" one. You hide yourself with Dark Harvest and Duskblade of Draktharr lock and ready, you shoot, they die.

Not gonna waste your time here, i'll explain quickly:

Infinity Edge better than Essence Reaver in 80% of the games.
Berserker's Greaves better than any other boots, bruh.
Rapid Firecannon is best but other are all more than ok, just pick only ONE of the atk speed + crit items, you NEED those build slots free for other items.
Duskblade of Draktharr better than Youmuu's Ghostblade in 90% of the games.
Bloodthirster or Mercurial Scimitar, is up to you if you can actually press an active items. Not joking here. Death's Dance is meh but suitable.
Last Whispers up to the game, TANKS= Lord Dominik's Regards, no tanks= Mortal Reminder