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 get killed by an enemy, press the taunt button. If you want to do it quick you can just run into the enemies and die, but if you play the game enough you surely will have some deaths, you just have to remember to taunt.
Don't forget, there's also an Achievement for taunting after you kill an enemy Naut.