How do I win?
Well, first of all, I play this:
|Double +1/+1 is sooooo good!|
Then I play this:
|It doesn't really matter who on.|
Then it really depends on my opponents life. If they're still sitting at 20, then casting its Flashback on the same (or even a different creature) is a good idea, seeing as it'll bring that creature to over 20/20. If they're around 14-15 or lower, depending on the creature it was played on, then there's no need and we go straight to the next step!
|"You can't catch me, fatty!"|
Then we attack with everything and they die. Game over.
So let's check it out step-by-step quickly. We play the Corpsejack Menace, who doubles +1/+1 counters placed, then we play Increasing Savagery which normally would get me 5 +1/+1 counters, but because of the afforementioned doubling, we get +10/+10 on a creature (let's just say it's the Menace for now), so we've got a 14/14 sitting around. This is pretty pointless if they can block him though, isn't it? So we throw down a Golgari Decoy and attack with both. They're now forced to block the Decoy and your Menace goes straight in to do 14 damage.
By this point it's usually enough to finish them off, but if it's not then just spend another turn to Flashback the Increasing Savagery and make him 34/34 for some great overkill. Or just Scavenge a whole bunch of creatures if you use them (I'm fond of Slitherheads personally, and 3 of those would get you the 20/20 you need for absolutely no mana cost).
With my limited experience with the game, I'm pretty happy with this to be honest. Now I've just got to work out the rest of my deck...