You can find Bonus Puzzles by solving certain clues in the main game.

 

Tap the 'Bonus Puzzles' button at the top of the home screen to view the puzzles you've found so far. 

 

You can have up to four Bonus Puzzles available at a time. If you find more puzzles, they're saved to your collection, ready to fill an empty slot when you finish another puzzle.

 

Bonus Puzzles take time to unlock. Only one puzzle can be in the process of unlocking at a time. If you don't want to wait, you can also use tokens to unlock the puzzle immediately.

 

At the bottom of this screen, you can see how many Bonus Puzzles you've found, completed, and the total in the game.