You will reach the trident fort in chapter 4 as part of the games story, so you can't miss the place. You need to get to the door at the end without alerting enemies. You can kill them if you wish so long as other enemies do not spot you in the process. It is ok if 1 enemy finds the body of another, this will make them turn yellow, but it doesn't mean they have spotted you.
If you are having trouble, beat the game once then come back here on explorer difficulty and use the silenced pistol to clean up and get through this really fast.