You can configure your control buttons in the settings, there you can alter what button you taunt with. The standard button for taunting however is up on the D-Pad.
Play a match, bots are allowed, and as soon as you kill an enemy, either a bot or a player, press the taunt button. You have to kill an enemy Awesomenaut, so the little critters don't count. If you just play a bunch you'll get some kills and plenty of chances to taunt, but you can also obtain this by playing against a friend.
Don't forget, there's also an Achievement for taunting after you get killed by an enemy.