Your ship stands on the left, while one of the enemies stands on the right, with you using the mouse to aim and shoot its cannon, using the finger to drag and release if playing on a mobile device (phone or tablet).
If you shoot the enemy enough to deplete their health bar, they explode and you win, advancing to the next battle, which is always more difficult. Usually, that is because obstacles are being put between the two ships.
When life or other upgrades fall from the sky on parachutes, shoot through them to collect them and maybe use them to your advantage. Well, not maybe, make sure to take advantage of them!
You get points in return for the battles you've won, and you can use them for upgrades as well as for buying whole new boats to use, of course, which will help you in the battles that are more difficult.
You choose between the Easy and Hard levels of difficulty, and then you can start giving the game your best and having lots of fun! Enjoy!