To start the mission, simply SWITCH to Franklin. He will be standing near the Mile High Club in downtown Los Santos. From there on, the player has to follow the architect without getting too close, and once in the building, knock him out and obtain his briefcase.
Then, drive back to Darnell Bros. where Michael and Lester will be waiting. A cinematic will show both available approach for the heist, and the player has to choose either.
Use the janitor uniform and ID badge to enter the FIB building at night and plant bombs on the floor where the hard drive containing the required files is stored. Once outside, detonate the bombs.
Then, disguised as firemen, show up and get to the damaged floor to get the hard drive and walk out before the real firemen show up.
Choosing the fire crew approach will unlock the following setup missions :