Getting Inside the Facility
There are six guards out here. There are three on the left and three on the right. If you wait for them to split up, you should be able to do a takedown on the first guard to the left and then chain up the takedowns to clear the left side. Put a tranquilizer round into the door guard and the way is basically clear.
A slightly better option is going to the right. Take out the first guard to the left now and hide the body by the shack, then cut to the right. If you hurry, you should be able to make it to the lone guard standing behind the truck. If you take him down here, you won't be spotted. If you look at the office to the right, you should see some well placed crates. Just grab a box and drop it on the ground to climb up and go through the window. Loot the room and open the door to come out right behind the guard. Take him down, and shoot the guard on the raised ramp with a tranquilizer.
Move back to the left and you can easily take out the stragglers.
Now that the yard is clear, we can carefully consider our path.
There's one that goes around the back, but it seems a little needlessly complicated. There's also a rooftop path. The front door is also open, but you really don't want to end up on the first floor. It's a bit of a killing field.
The best way is to hack the keypad on the gate to the left of the front door. If you let Sanders go, then you already have the code. You learn that 7984 will unlock most of the keypads, and "Neuralhub" will unlock security terminals. Regardless, it isn't a terribly difficult hack. Open this up and climb the ladder to find a route into the main room and you'll wind up on the second floor.
If you can't get in through this route, try going in through the front door and then use the crates as cover to get to a ladder and make it to the upper level. It's a really good spot.
Once you're on the upper level, you just need to watch out for a sniper. He's directly across from you. Just stay low and walk toward him. It's fairly easy to just wait for him to start a patrol and quietly take him down. You can also grab the gun if you want. It's big though, and it won't do a whole lot in this area.
The real treat is right next to the sniper's perch. If you look by the sniper, you should find a vent that you can go in, which will put you out right next to the elevator that you need to take. If you use the vent on the second floor, you will easily be able to slip behind the elevator guard, hit the button and then go down to the holding cells.