This is really simple.
Walk over to anyone and melee them by pressing
.
Pick the body up by pressing
.
Carry the body to your car and press
.
Achievement done. No dart gun needed.
An easy opportunity to do this is during the second mission when you arrive at the shopping centre, as seen in the video below, but you will also do this when completing the Bounty Missions, so you don't have to go out of your way for this.
EDIT: my game clip has vanished so the YouTube link is someone else's.