After finishing up Cleaning Out the Bureau, play as Michael and you'll eventually get a phone call from Franklin saying he's tailing the architect. This will make you automatically switch to Franklin, who will be outside a construction site. Head inside and follow the architect, taking the elevator when needed.
Once he gets to a certain point, sneak up behind him and knock him unconscious. Snatch the briefcase and leave the construction site the same way you entered, as you're pretty high up and will most likely die if you fall off. When you're clear, grab a vehicle and head back to the garment factory to finish the mission.
When you deliver the blueprints, you will have to choose between one of two approaches for The Bureau Raid. Depending on which option you choose, you will have to do some setup missions. The Fire Crew approach will require one extra mission and the need for two gunmen to be hired.
The other option, the Roof Entry, will require no extra missions, but you will need to hire a hacker, gunman and driver. Whichever way you choose, both walkthroughs will be listed on the same page.