Firstly make sure you have over 300 Floins, there is a counter for them in the lower left of the screen, then hold the
button and select the coin bag icon lower left next to the fist icon.
Providing you have the correct item selected you will see the option to 'Throw Money' on
in the top right. Press
until you have thrown over 300 Florins (10 Florins per press) and the achievement is yours.
Here's a video.