Kogmaw is GREAT to fight early, and gets stronger and stronger through the game. If you want, you can play teleport to make your powerspike come true faster!

You have to be careful about champions that are stronger than you early on like jax, warwick, irelia or yone on top, zed, diana, akali, yasuo or mel on mid, lucian, tristana or nilah on bot
and support is also important, and this changes things a bit.
If you have good synergy support (like those in the guide) and this support wants to play aggressively in early, THEN YOU SHOULD TAKE THE FIGHT NO MATTER WHAT!
If the other supports are on your lane, then f*ck them, play your own game, fight when you are ahead, scale as fast as possible!

About items mentioned in this guide...

First item is ALWAYS rage blade!
Because of Kog'maw's passive it's just too op, you might want to buy socks while building guinsoo if you are much ahead and you don't have enough money for an item or if you are a bit behind and you need it to dodge enemy spells like morgana's Q.

Then after first item, you buy boots and decide which one you buy.
Depending on team draft (if you are playing top or bot, but see that you have fed zed on mid and the rest of the enemy team has more AD damage, then you might want to buy Plated Steelcaps.
Same goes for AP damage with Mercury's
AND IF YOU HAVE GOOD SUPPORT or you are too fed to buy defensive items first then you might want to buy berserker's greaves!

Second item depends on many things like matchup, state you are in, and many others...
I'll mention only matchup and state cuz other things you are going to decide yourself!
You might want to go BOTRK when you are too fed to play Runaan's Hurricane, but its still good.
When you play against tanky top/mid/support then you also might want to play BOTRK!
But when your matchup requires safe gameplay then you'd probably better buy Runaan's Hurricane first! It will help you with farming even with harder matchups, harder early game, but as I said it is also worth buying as a fed badass in rather good matchups cuz it might speed your powerspike to maximum!

As for the third item it is Blade of the Ruined King or Runaan's Hurracane depending on second item choice!

Then you go into late-game items...
Those are very important. If enemy team has a lot of armors, then you might want to buy Terminus into Jak'sho! It makes great combo! And with this choice you might want to replace your boots with Titanic Hydra because it will increase your damage a bit and your AOE dmg will grow a lot! Also the item makes you almost impossible to kill!
(well if they stune you, then you are probably still dead, not always but you know how it goes...)

For the other one situation when you have not much tanks that are building up armor and mr, then you might want to drop Terminus out of the build and buy Jak'sho first, then Titanic Hydra.

Good supports with great synergys means strong first fights with a lot of damage.
If you take teleport then you don't have to be scared of the outcome of this fight.
REMEMBER! Good occasion gives you the opportunity to scale faster! And teleport will help you with not losing that much after the fight.

Bad suports means safer gameplan with mainly farming to the first item. If you get your first item before your enemy does, and they are slightly behind with exp.
Then you might want to even ALL IN! (also depending on a support but if enemy adc or supoort is alone, then you probably will win anyway!)

As for the top and mid...
If you play against someone who loves to scale but is pretty bad in early game, then ALL IN! Even if you die, then you have teleport to save your exp and cs and if you get the kill or two, then you will become pretty much unstopable!
If you hit the wall like jax, yone, kata, zed, diana or other good early champions with strong oneshoot potential, then you might want to cut them off the farm and collect your cs while freezing the wave! That prevents them from getting essential gold and lets you hit them almost for free!