Both of them are easier on Recruit, obviously.
In the mission “Dark Water” I’m pretty sure it’s the first set of stairs shortly after the mission starts, when you finish clearing the bottom deck. As you get to the top of these stairs 2 porta-potty’s can be found. Shooting into the left one will net you the achievement. get up the stairs a s quick as you can, and as soon as you see the porta-potty, shoot it. If you take too long the enemy will leave the toilet and the AI will kill him, but you can easily restart to the previous checkpoint to try again.
The second one is a little trickier, but on Recruit it should be easy enough.
‘Ghost in Training’ has you having to reach the penthouse in El Sin Nombre undetected. For this achievement, once you have a mask you can have free roam of the main floor and outside of the house. Walk around to the backyard, until you see the little balcony above the garden walls. Climb up on the wall, then the balcony and through the door…
Go prone and climb up the stairs, grab the bottle on the staircase (and any other bottles you come across), crouch walk down the hallway towards the right (towards the statued), take a right and open the door. Hop over the railing outside and onto the roof and to your right you will see a ladder.
Take the ladder up crouch near the peak, right on the left of the little house-like structure. Wait until the guard straight ahead walks from right to left and onto the other roof. Now, you wanna crouch walk up to the second bottle straight ahead. Turn to your right and throw the bottle to distract the guards. Quickly crouch walk ahead and see the guard to your right distracted. Walk behind the grills, throw the second bottle at the same spot as the first one and continue past him and on to the next ladder.
You will then meet up with Alejandro on the roof and proceed to the elevator shaft. Navigate to the 3rd floor and you are home free. Walk up to the door and the achievement should pop!