Main rule: Kill your own worms; each round, whoever achieves this first wins it.
In case of a drop, you have to rematch the dropped round, following of course the team order from previous rounds. If your opponent doesn't come back within the next 15 minutes, then you proceed to the next round.
Maps: Any open map without bridges (and preferrably with a non-grassy texture) could work. Best ones are semi-complex or complex, and don't have any "blocks" that could penalize either of the players very hard.