Click the button to begin playing. You can pause the game at any time with the same button. It will pause automatically if nothing happens for a while.
Four words will appear near the top and you will hear one of them. Click on or near the word you heard to "zap" it. You can listen again to the last word by clicking the replay button.
The monster moves about and sometimes gets in the way, so you may have to wait for the monster to get out of the way to zap a word. If the monster annoys or distracts, you can put it to sleep by clicking the monster button on the left panel.
You can make the words (on average) easier or harder by changing the level. This can be done at any time including when the game is in progress.