When you start the DLC and arrive on the planet, start by sabotaging the door, then make your way towards the bridge. The controls on your level are broken, so you have no choice but to get down using the left stairs and getting noticed by the Varen. Shepard will probably even tell you so, but fear not for your stealth-related achieve, because Varens do not count. You can always try to use the Cloaking perk if you play a sniper to kill the Varen, but honestly it is a waste of time.
Activate the controls, go back upstairs and cross the bridge. Progress until you have to shoot the gas pipe. Do so (releasing another Varen), and manipulate the valves so you can access the door ON THIS FLOOR, on the right corner. Do not go for the upper door just yet. The valves you need to turn are the one in front of the door and the one at the end of the left hallway. In this "secret" room, you will find your first upgrade (ammo increase for your heavy weapons) and some cash in a box. Manipulate the valves again and go for the upper door. Nothing of interest here.
When you arrive outside, I would suggest you saved. Walk straight ahead, the 2 guards on the left won't see you. Kill the power lines, then make your way up the truck using it's rear (with the tail lights). DO NOT approach the guards, or you're toast. Go inside the door, take the cash in the little box, then go right. Do not open any door, of course, or you will find the guards. At the fork, go left and explore the back room for some Iridium, then get back on track and jump down (no turning back now !). Go up the stairs, hide right next to the open door with the 2 guards (ammo behind you), and SAVE. If you go for the collectables in the room, you will get burned. So SAVE here so you can reload later, after you get your achieve, if you want to get every item like me
In the room, you will find cash and an audio log. Now get to cover on the left of the door, and you will see a guard going into a distant door, patrolling. Do not be scared. As a matter of fact, follow him and enter the door when you see an air patrol landing. You can hide behind the railing when the ship lands, but it's not an obligation. Inside, you're alone. Activate the crane, then the elevator. Go down again. Activate the elevator again for a datapad and a health pack. Take the door, the cash on the left and cut the lasers. Now stick to the right wall and the left guard in the small room (which is empty) that is staring right at you won't see you. It's kind of stupid, but it's the case. So continue down the hall, turn left and open the door.
Achievement unlocked AFTER THE SCENE