This mission is given to you by Lester (marked as "L" on your map) and is available to Franklin after finishing Hotel Assassination. Make sure you have a fast vehicle, or even a helicopter, for this mission, as it is timed and you don't have a lot of room for error. There will be four different targets, spread out across multiple locations, and you will need to kill each one.
You will have a total of 9 minutes to kill everyone. The first guy is a bodybuilder and it might be a bit confusing, since there will probably be more in the vicinity. You want a big guy who is likely flexing his muscles, so shoot him and then head to the next target. This one will be on a yacht in the waters, so there shouldn't be any confusion as to who to hit.
The only problem with this one might be the waves making the yacht move up and down, making it slightly difficult to get a good shot. The third juror is no problem at all, as he is washing windows on some scaffolding on the side of a building, making him an easy target. For the last juror, he will take off on a bike when you get close enough. Build up speed and ram the bike to knock him off, then run him over for the kill and to end the mission.