Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Kog'Maw is a beast. The biggest choice to make is whether to go spellpower, or attack damage.
I personally love to get him attacking and moving quickly, doing a bit of lifesteal along the way. Things are just better when you can duck through the jungle, and refill your health on creeps.
This build emphasizes movement speed, attack speed, and attack damage. You will be staying at the edge of the battle, and rushing in for a kill only when it's safe.
The vamp scepter is purely for early laning. Later on, try to balance attack speed, damage, and movement speed. The item build is highly dependent on who you're up against.