Washers is a game that can be played individually or in teams. However, each player should have his/her own set of three (3" diameter) washers. Each set of three washers should be painted a different color for easy identification in play.
Choose the order in which players will throw. The first player will stand on one box and toss the three washers (one at a time) toward the other box. Sinking a washer into the hole closest to the shooter is worth one point, the middle hole two points, and the furthest hole three points. After the first player shoots all three washers, the second player throws their three washers, then the third player and so on. When that round is completed, the players move to the opposite box and collect their washers and count their points. The player that scored the most points in the round will go first, followed by the second highest scorer of the round and so on.
The game continues until any player (or team) amasses 21 points exactly. Should a player go past 21, they'll revert back to 18 and continue. In other words, if a player is sitting on 20 and tosses a two, they'll add one point to get to 21 (which then reverts to 18) and add the second point to give them 19.
Keep in mind that it is always beneficial to go as early as possible in every round. Any washers that don't make it into a target hole but are left on the board may be knocked into a hole by another player, so the last player is at a disadvantage.
Every player seems to have a different tossing technique. Some try to throw the washer as flat as possible with a low arc so that the washer lands and slides across the board. Others prefer to arc the washer high and end over end with a bit of backspin to try and coax it into the hole like a basketball. Practice and experimentation will help you find a rhythm and method that suits you.