Depending on your choice at the end of Architect's Plans, you will have the choice of one of two approaches to this mission. If you choose the Fire Crew approach, you need to have set a getaway vehicle, as well as finish Fire Truck. The other approach, Roof Entry doesn't require any extra missions.
Both walkthroughs will take into account you hiring the best people for your crew (usually the ones with the highest cuts).
Once you gain control of your character, get into a vehicle (waiting for the other character) and then head to the government facility. Get in the chopper and fly it to the marker in the sky, where you will jump out of the helicopter. Your target is the top of the FIB building, so freefall until you get closer and then open your parachute, guiding yourself to the top of the building.
After you land, a cutscene will bring you into the building, so go to the southeast corner to the server room door. The door is locked, so switch to Sticky Bomb and plant one on the door, detonating it from a safe distance. Once inside the room, follow the instructions given until you get to the hacking portion.
You only have so much time to find the IP address, which is written at the top of the screen. For the next part, you need to hit the designated button when the highlighted letter reaches the middle of the screen; if you miss a letter, then you will need to start from the beginning of the word. With that passed, you can now start the download of the file.
You have 2 minutes to fight off some enemies while the file downloads, and you can switch between Franklin and Michael here. Don't forget that enemies will also appear on the roof outside of the skylight, so keep killing them until you see your chopper go down. Go back to killing enemies until the download is complete, then grab the device and follow the others.
Keep following the others, killing the enemies in your way as you go through some doors and down the stairs. After spotting the helicopter, there will be a small firefight and then more running and gunning. Go down a second set of stairs and there will be a few enemies, then you will need to rappel down the side of the building.
You'll end up on a balcony, where you will have to take care of an enemy helicopter. If you have a RPG, then wait until the chopper is steady to shoot it, or you can get a bead on the pilot and shoot him to down it. Continue to the next spot and rappel down the rest of the way to the ground.
There will be a few enemies around the corner and then it's a straight shot to the getaway vehicle, which is the Ambulance. Drive to Franklin's house in Vinewood Hills to finish the mission.