Go to the eShop, tap on the Splatoon Testfire banner, and download the Testfire game. (It's on its own page, separate from the full game.) Then run it during the Testfire sessions today, Saturday, and Sunday. There's one 2-hour session each day, the time depends on your time zone: 3:00 - 5:00 pm Pacific Time 4:00 - 6:00 pm Mountain Time 5:00 - 7:00 pm Central Time 6:00 - 8:00 pm Eastern Time