Hit jobs labelled "Easy". You might have to click "Show All" to view "Easy" jobs. Every successful hit on the job means a 2% chance of an extra credit dropping. I suggest unloading on the first job and you might rack up some extra credit in no time. Extra credits will drop randomly, you might get EC on one unload and no EC the next. You can also beg, I mean sweet talk, your relationship partner (if you have one) for extra credits. Check out this Extra Credit guide by Jerk if the above does not make any sense.