NOTE: First REAL guide, so please bare with me...I'm doing it for everyone else largely, my Hecarim is pretty op.

Hecarim is a late game champ, this has it's plusses and it's minuses. On the upside, end game is your friend, on the downside you have to get to that point, and you MUST jungle. Games that last 40 minutes are hugely in your favor, so avoid surrendering.

When jungling you don't do enough damage to initiate ganks, so wait for your teamates to start the fight, and then show up to knock back your enemies with the charge and finish off the kill (if at all possible).
...everyone will hate me for this, but early game it'll help you a lot if you ks a couple. I'm sorry but it really does help...just don't be stupid about it.
Start at blue, and then go a standard route from there.

Don't surrender. Depending on your team, chances are possible early game will suck...idc if everyone and their mother is trying to surrender. I had a mid game where I was 7/1, EVERYONE wanted to surrender, but I thought I'd stick it out...end game I was like 26/3, we won.
This is something else that will annoy the heck out of your team, but if you end up winning they'll thank you (not really).

You need wriggles first for jungle, but it is a bit of a set back, I've had games where I stuck with the razor and held off on the actual Wriggles.
You need Sheen for massive damage output, after that TF, and once you build into the Giants belt, you're OP...and you should begin to notice it.
If you actually finish up your build, you should be unstoppable, your last item is a toss up, and I'll leave that up to you, although I put down FoN for reference.

Thanks and I hope you enjoyed my guide.

At the moment I'm just throwing out the quick reference tips listed above for pros, I may go into more detail later.

P.S. Due to the fact that Atmas Impaler has been nerfed, it may be worth picking up Bloodthirster in its stead.