Before the mission
Meet up with Steve and Lance at the Printworks. You are going to tell them about your plans to hit the courier carrying the printing plates and you are going to bring them back to the Print Works.
After the completeion of the mission cutscenes, you will be given the following instructions:
The counterfeits plate courier is arriving at the Docks in a helicopter any second now.
Get a fast car and then quickly drive to the meeting in some docks at Viceport. There you will see the receivers, a group of armed women. Pull out the Sniper Rifle or Combat Sniper rifle and snipe out all of the women. Once you have done so, run out into the open and wait for the helicopter to touch down.
Now quickly, pull out an assault rifle (since she's wearing armor) and gun her and her escort down, before they reach their getaway car, since if they get on the car you will have to snipe the courier dead, because if you blow the car up with the courier inside, the plates will blow up as well, resulting in mission failure. Once the courier is dead, pick up the plates and run for it, then get in your car and drive away, now just drive back to the print works and deliver the plates. Mission accomplished!