How are winning, losing and saving pitchers decided?

At the end of the game the scorer decides the winning and losing pitchers under Official Baseball Rule 9.17, and any save under rule 9.19. Points from the CABS study guide:

• A starting pitcher must pitch 5 innings to earn a win (4 innings in a 5-inning game). The "must pitch 5" condition applies only to starters: a relief pitcher can get a win after facing one batter.
• A loss can be charged to a starter who faces only one batter.
• When a game is tied, the contest for win and loss begins again.
• The key to finding the pitchers is to find the game-winning run. The guide suggests pencilling in W/L/S credits at each pitching change.
• A save is ordinarily given when a closing pitcher maintains a winning margin in a close game or pitches at least the final 3 innings. A save is not always applicable, and can apply even when a reliever enters with a lead of more than 3 runs, depending on inherited runners or closing the game with 3 or more innings in relief.
• Juniors do not ordinarily have winning or losing pitchers, but Under 18 may.