Step one:

Cut the handle off a cheap plastic axe/trident/other Halloween costume prop, or otherwise acquire a length of thin tube.

Step two:

Duct tape one end of the tube to an air bladder of some kind. The bladder must be fairly durable. I used one from an old "stomp rocket" kit.

Step three:

Go outside, push a dart down the tube, angle it slightly, and stomp on the bladder.

Step four:

Lose sight of the dart because it went a lot farther than you expected it to. Whoops.

(Follow just steps one and two if you want to know how to make a cheap yet effective Nerf mortar instead of how to lose darts.)