At the beggining of the game make suro to ward the opposite buff, for example u bath top to bot make sure to ward your bot buff if you start at the top.

ealy game ganking: after poping your ghost and e make sure to get an amazing gank angle (push enemies with your e as close to your allies and as far as possible from their turretsPhase Rush Guide) to help your lanes get as much abilities and hits on the enemy champ

a lot of jg mains ussualy just start or get oracle lens after first back, thats wrong and beacuse of that after the 4th minute ward an objective so you can see if enemy is trying to take it or not, ask your laners to ward opposite objective. either contest or trade objectives for example xinzhao is on the drake, you see that you have a weaker bot losing mid and you know that xinzhao counters you so you just go take voidgrubs and trade objectives

jungle fundementals
if a laner steals your kill, take your herald eye or smth like that, you have to tax them by either sharing,taking or take a wave on their lane.
dont ignore other lanes and if possible after mid game split push if needed
if you have a strong mid laner you can always invade both sides of enemy jg that goes aswell for your top but he should only help you invade top side same goes for bot
in order to invade after early game you should always try to see where enemy jg is and thats his worst nightmare, for an example if enemy is taking drake while your bot is dead and you know that his raptor are alive you can always invade take his red side or camp with your toplaner or midlaner in his jg and kill him for easy gold and then take his camps

objective scaling that you should focus on
Elder
Drake Soul
Baron
Infernal Drake
Mountain Drake
Second Voidgrubs( really valuable after getting shojin you can kill them easily make sure to either set enemies behind adn not let them get second voidgrubs or take firth and second so you can hel you lanes a bit)
All Other Drakes
Herald(no plating no easy gold)
First Voidgrubs(can be more worth then drake if enemy has weak jg or is taking drake so you trade and get both voidgrubs)

With a lot of new items hecarim changed but in a much better way, instead of spamming q hecarim can just execute enemies without trouble

For example our full build is very deadly and can be used as death itself ionian boots: we get ms and ability haste our ghost also reduces its cooldown making it more spamable.

shojin: wit the neew change we only get 20 ability haste and 15 ability for non ultimate abilities instead of ability haste scaling from our ad. shojins hp went from 500 to 300
but its ad is greatly incresed on abilities (ability damage can be incresed up to 4% for 4 stack meaning that we can have 12% incresed dmg for abilities making it a perfect one shot item for hecarim and many other.

opportunity: bassicaly youmus old passive that gave us lethality when not in combat and it dissapears 3s after initiating combat with a enemy, the item gives us 5% ms and massive amount of ms ater killing a champion and it also has a good build bath

profane: the most broken item for hecarim it gives us everything haste, dmg, lethality bassicaly evething you can ask for it also has a active that helps us execute enemies below 50%hp

seylda: it gives us armor now but its aight, our armor pen now scales with our lethality and only slows enemies that are blow 50%hp

youmus: heavy nerfed but still a decent build for heca only use before pressing q it also gives more ad to your profane hydra and shojin so it synergizes good with your items

how to use the build??
when iniating combat pop your ghost you e and wait for the time to activate your youmus, once the enemie champ is below 50% use you profane hydra.

when 3-5 man ulting you can use profane hydra right away since you can get insta shoted by some ap champs

Ill let you pick this one, sometimes youmus or serylda arent the best choice since you can risk of dying to early and that 2s of survivability can somtimes be game winning so you can always change your final 2 items or your boots

merc threads: alot of cc,slow or ap dmg
plated: a lot of true or ad dmg
cleaver: immobile enemies with low armor but can still one shot you
randuinds: perfect items against crits and highly mobile champions, can be excellent as a final item and its active is excelent for teamfight and a good item to use before passive from ur serylda procs
jaksho: good beacuse it gives you enough armor and mr for those extra 2-5 second of suvivability as a front liner sometimes if you survive long enough you can finish the fight by ur own
frozen heart: good against high as champs that are mainly champs like yasuo yone etc
serpant fang: break enemies shield only situational and rarely used
iron solari: good if you have 3 close range champs to tank with you on the fornt line
cahinsword: antiheal
Edge of night: can be ok if you wanna engage and not be perma cc right away
deadth dance: aight item if you need any extra ad and dmg at the same time only build it as a last item
maw: too much mr for example fizz veigar or syndra that can one shot you

for example we can play against draven nautilus gankplank lilia and yasuo
so our build could be
plated steelcaps, shojin, opportunity, profane hydra, serylda, randuins and thats bassicaly counter item picking the enemy team

Keep in mind that core build can always be changed for example you shouuld never go serylda against zero armor. muramana or edge of night are more valuale that serylda against squishy comp