This is pretty easy. Keep an eye out for someone sweeping. People sweeping with brooms appear randomly. Especially in the Centro district. They seems to appear only during daylight that I've noticed. Once you spot one, do something to make them drop the broom like run into them, punch them, or kill them. Make sure you're unarmed. If not, just hit
on the D-pad to be unarmed. Then press
to pick up the broom and then head to the nearest unsuspecting guard you can find and just hit
to kill him with the broom.