This is an AD Kog'Maw build, in which I focus on AD to such an extent that I end up with no AP. If you disagree with this idea, this isnt a guide for you. I combine AD, lifesteal and AS to enssure not only high burst damage, but high survivability and profits with the ability to heal by farming minions.

The items I chose for this build are targeted for a player up against a team with at least one tank, so that Madreds' passive will stack against Kog's 'w'. If the opposing team has no tanks, exchange the Bloodrazor for Wriggles, which will get you through the build faster without wasting gold on an innecessary item. If you do this, though, get the phantom dancers before the bloodthirster, to compensate for the AS lost without Madreds.

For this 21 0 9 mastery build I chose masteries to maximize AD and mobility. With them I get AD, AS, armor penetration and lifesteal, key stats for this build. In the utility branch, I get masteries to ensure greater survivability and mobility to account for Kog'Maw's inevitable squishiness.

I choose Exhaust and Ignite as Summoner Spells for various reasons. Combining Exhaust with high AS ensures a high amount of hits, meaning that you can get an easy first blood and more kills in early game. Ignite works extremely well with Kog's ult, as you can Ignite while being unsure of getting the kill, because if they hide or turret hug you can just ult.

Kog, specially with this build, benefits extremely well from crowd controls which allow him to land extra hits, like slows or fears. Because of this, you should ask for ganks a lot in early game, to get fed and advance through your build, or gank lanes with good cc'ing champions. When your target gets cc'd, DONT just attack. Try to land both your 'q' and your 'e', and try to get close to land your latter on their escape route. This will maximize the amount and quality of the hits you'll land.
When in team fights, if a tank is present take advantage of your 'w' and Madreds Bloodrazor and take them down as fast as possible. If the opposing team has no tank, or he isn't present, focus on the squishies, as with all your AD and AS they will go down fast.