Only use a cinder block and bring it to the end of the mission. If you switch to another cinder block, the achievement/trophy voids. If so, restart the mission to try again. In certain parts, you'll have to leave it behind including, and without limitation of the following:
1. Shooting any enemies
2. Running certain objectives like planting and detonating C4.
Maybe possible on any difficulty, but Recruit is recommended. Achievement/trophy pops if successful.
Note to self: You also need to kill every enemies in the field of dead bodies. Some players have reported that the achievement/trophy for "Play Dead" have popped by having Farah kill some enemies. It may have happened to BradLeafsFan9, but I personally recommend killing every enemies to be sure you got the achievement/trophy.