Build a bridge that is at least 50 units long.
Bear in mind that it will be impossible to do this on some maps. If you're specifically looking to get this achievement, make sure the map has a large stretch of water when you start.
Ensure that you build up a large supply of logs for building the bridge - the minimum amount of resources required will be 200 x logs and 50 x stone.
There used to be a bug that caused builders to walk to the other site and starve ("the march of death"), but this was fixed in a later patch.