28 Nov 2018 28 Nov 2018
While progressing through Adventure Mode there are opportunities to unlock Bonus levels, depicted as small circles slightly to the side of the main path. Tap a Bonus level's circle to see the requirements for unlocking that bonus level (such as "Get 3 Stars on Puzzle 10").
On the main Adventure Mode map, the first bonus level you will encounter will be at Puzzle 10, however, there will likely be earlier opportunities to unlock a bonus level in the Extra Maps. From the Adventure Mode panel, click Maps in the upper-right corner, then the Extra Maps tab.
I found that the June 2017 Extra Map had a Bonus level after completing Puzzle 3, though this Extra Map may end up cycling out of availability at some point. In any case, check the various extra maps if you're looking to get this achievement as quickly as possible.