The goal of the game is to use your flail to knock your opponents into the spikes at the edge of the screen. The last one to survive wins. Longer swings of your flail knock your opponents back further than shorter swings. You can also throw your flail, but this leaves you unable to pull a new one out until the thrown one leaves the screen. In addition, you can use your flail defensively to block attacks.
Many dual analog controllers (such as Xbox controllers) will work with the game. A full list of the controllers that should be supported can be found here. Here are the controls for these (the diagram shown is an Xbox controller, but controls for other controllers should be laid out the same, even if the button names are different):
I highly recommend using controllers, but if you don't have any, the keyboard can also be used (but it's currently limited to 2 players). Here are the keyboard controls: