Chapter 6: Welcome to the CIA
This chapter is a series of three different types of training, one of which is stealth. There are two stealth sections, the first of which is a tutorial, the second of which is your final CIA training mission. It is the second which you have to pass without being spotted.
Assuming you have saves turned on, there is a save point right before this training starts, so you can quit and reload if you get spotted (it'll be really obvious).
Strategy wise, move from hiding spot to hiding spot, taking out soldiers when they get close. Just watch to make sure they're not looking toward you before you move. For the first one, you can use Aiden to lure him to you. When you get near the last two (one marching around the flag, the other up high on the right), the easiest approach is to head right and up the ladder and take this one out (you can also use Aiden on him). From up high you can then jump onto the last soldier to get the flag.
Ryan will "see you" at the end, but this is not a fail in the mission and the trophy will unlock as the next chapter loads.