Main rule: Kill your own worms; whoever does it first in a given round wins it.
In case of a drop, the dropped round has to be replayed, keeping in mind the team order that occured until then. If your opponent doesn't come back (or is absent altogether) within the next 15 minutes, then you proceed to the next round.
Maps: Any open map without bridges and with a non-grassy texture could do the trick. Best ones are semi-complex or complex and don't have any blocked off area that could penalize either of the players.