Pants are dragon is a challenger player on the NA server that has a
Youtube channel where you can watch him play Hecarimon on his "Challanger to rank 1" series to better understand what he does to be the very best.
He is also the #1 Hecarim on Lolskill.com in NA at the moment.
https://www.youtube.com/user/luigidragon
http://www.lolskill.net/summoner/NA/Pants%20are%20Dragon

pros

1.Good early-mid game ganks
2.Can basically build anything(BUT AP)
3.He is not useless when behind

cons

1.Scales badly into late game
2.Farming jungler
3.Not the best fighter
4.Mana hungry

Starting:



If you are playing against a champion such as and want to avoid being counter-jungled you can upgrade your to to stay Full hp while farming the jungle .

Rush trinity + Boots:



At your first back get and or just if you have the money and then buy at second back with .

Finish jungle item and Boots:



Start with into then finish or depending on the enemy team.

After core items:

After you have finished your core items you usualy want to start building and (if you need MR start with and if you need armor start with ).

: You want to rush this item as 4-th if they have champions such as .

: Very good item if you want to engage a lot.

: You can build it if you are very ahead and you want to make sure you carry the game.

: Can be built instead of if you are against a or you want more AP.

Peel items:
You want to build these items if you have a fed adc or a hypercarry such as that scales very well into late game.

: Best choice if enemy team has a assassin or a high burst champion that you want to protect your adc from.

: Works best if your team already has some engage so you don't have to engage and can stay close to your adc.

: Good against poke champions or champions that deal lots of damage over time such as and .

: Can work if enemy got a and your support isn't building it but usually its a very bad item to make.