vizthex90,472 14 Sep 2023 14 Sep 2023 14 Sep 2023
In the level menu, the bonus levels are the ones with the cubic icons near them at the bottom of the page (while a red flag means it's been completed). You'll get a little crown on the world track on the top of the screen when they've all been completed.
They unlock as you progress through a world, so I would advise you get this at the end of the game or each world.
Just finish each level and you'll get the achievement.