The decade is based on when the first studio album was released.
Start by clicking an artist who first released an album in the 1990s. Next, click an artist who first released an album in the 2000s. Then click another artist who first released an album in the 1990s, then an artist who first released an album in the 1980s. Repeat the process 8 more times until there is nothing left to click.
In order to create a playlist on Sporcle, you need to verify the email address you used during registration. Go to your Sporcle Settings to finish the process.