At the start of "The final test" mission, go left of the entrance and climb over the fence and hide behind some boxes. Youll be able to overhear a conversation between mechanics. Listen to it, the game will tell you to push select to follow this opportunity, so press select.
Youll need to get a mechanics uniform, a wrench, and then press a button on the plane, the achievment should pop once you press the button.
You can find a lone mechanic in a nearby small room and a wrench is right next to where you press the button.
Id suggest following through with the opportunity for the K-36D achievement, which is gotten by eliminating the target with the plane opportunity. After you press the button on the plane, walk upstairs to the target, press Y to talk to him, he will follow you, and walk back to the plane. Once back at the plane, there will be a marked clipboard that will start the test.
The game tells you what to do throughout the mission so it should be pretty easy and also be sure to manual save.