Pretty simple. Go to the gang's hideout, get insulted, kill everything inside, and return for payment. Needless to say, it's dangerous to take on these runs in the early stages of the game if you don't have good equipment and a teammate or two to help you. If money's not a problem, though, and if you can take on four or five gang members with guns with relative ease, then these runs may be nice for building your Karma -- assuming you don't get the runs from Vigore and Jarl, of course. But getting 2 Karma for a quick job like this can be pretty helpful.