Simple to complete.
Go to season > custom > new custom league > Conferences/Divisions "off" > Teams = 4 > Add team method "1" > Press A and if you want to add a custom team of yours to make it easy you can but if not just make sure there are 4 teams then press B
Press "New Season" > Regular season games = 4 > Innings = 1 > Playoff games per series = 1 > Playoff teams = 2 > Select the team you want to be and press A
You can either play or simulate the games but I recommend playing them since they're only 1 inning they take about 5 minutes a game. Once you come in first place and the season is over press "Start Playoffs" for the achievement to pop.
If you play the 1 playoff game and win it you will also unlock the "Custom Captain" achievement. You have to play the championship game you cannot simulate it to unlock this achievement.