I probably won't finish this guide so I'm just going to post what I have and update it when I have time. Hope what's available is helpful enough!

I also have a decent amount of content in the notes above! Please read them!

Hecarim is a mobility-based champion that receives attack damage based on his movement speed. With his movement speed ((In this build)) it's not uncommon to catch enemy players off-guard. Hecarim will more often than not pull off successful ganks as well as very quick and efficient tower dives to secure a kill for the team when needed.

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I don't personally recommend Hecarim as a top-laner often in this upcoming season due to the amount of AD carries being seen in the top lane as of recent.

While AD carries may not be a big issue for Hecarim, their poke has the potential to change the game early on in the laning phases. The range on ADC champs allows poke while you have none for immediate return. This is NOT okay. Typically it should be a poke for a poke and if a skill goes off, then reciprocation should be the same. Having a champ like Hecarim who is not tanky by nature and a kit that is all-in or not-at-all, in the top lane versus a possible AD carry is a bad idea in general.

After buying your first items (( ^{Hunter's Machete and Health Potion})) at the start of a game you are going to typically want to get a leash for Gromp ((remember to use smite)) and move on to the Golem, wolves, and then raptors. If by then you have enough gold to buy your next tier of items (( ^{Stalker's Blade and more Health Potion})), buy them and continue jungling and start taking care of your laners if need be.

As a jungler

it is your explicit job ((among others)) to maintain central wards in the rivers, and enemy jungles, as well as gank any lanes that are having trouble or falling behind in the game. ((This doesn't mean others shouldn't help when they can and where appropriate))

Do your best to at least have a general idea of where the enemy jungler is and counter-jungle to deny farming and ganks for the enemy.

You CAN take flash instead of ghost in this build if you would like to. It's personal preference. Ghost is more helpful than flash due to Hecarim's passive. He gains attack damage from his current movement speed.
Since this is an off-tank build for an extremely strong late-game, ghost is more beneficial in dealing damage and catching up to enemy champs who may be running away. When paired with Hecarim's E his movement speed gets insane, and in turn, more attack damage in that moment.