Basically,
you choose odds or evens and remark "Once, twice, three, SHOOT!" each player either holds out a closed fist, 1 finger, or 2 fingers, therefore the total can either add up to 0, 1, 2, 3, or 4. If it is 0, no player wins. If it is even (2,4) the evens player wins and if it's odd (1,3) the odds player wins.