Not all ladder rungs are equal. The top rung is worth three points; the middle rung is worth two, and the bottom rung is worth one.
The goal is to be the first player to reach 21 points without going over. If you score higher than 21 – let's say you had 19 points, you tossed, and you wrapped a bola around the top rung, bringing you to 22 – the toss that got you there is discarded from the round. You keep retrieving bolas and tossing until someone makes that score.
More important, a bola only counts if it is wrapped around a rung at the end of the round. So if Blue ends up knocking his own bola off the ladder on a subsequent toss, that bola doesn't score any points. If Blue knocks a red bola off the ladder, that one doesn't count, either. So aiming for an opponent's bola is good strategy. (Aiming for your own is not.)
A red bola and a blue bola on the same rung cancel each other out. So if at the end of a round there's one red and two blue bolas on the third rung, Blue gets three points, and Red gets none.
(Variation: All bolas on a rung count, so in the above example, Red would score three, and Blue would score six. This can speed the game up.)
If both players make it to 21 at the end of a round, an overtime round commences. In overtime, there is no point limit, but a player needs to be ahead by at least two points at the end of the round in order to win. Overtime rounds continue until one player finishes a round ahead by two.
You can also play ladder ball in teams of two. It's not much different: Team members simply alternate tossing by round. You don't necessarily need two ladders for team play, but most sets come with two, so you might as well. It can save time, since there's no running back and forth for the bolas between rounds.
Plus, with one player from each team at opposing ladders, it can really raise the level of trash talk, which is critical in any lawn game. Spilling beer while celebrating is optional, but apparently, it's good for the lawn [source: Greenwalt]. (Who knew?)