All According to Plan... Achievement in Halo: The Master Chief Collection

As soon as the mission begins, head to your right near the cliffside edge and station yourself in between the “U” shaped rocks. Here, you can avoid being detected given the distance between yourself and the enemy.

Now you will want to pick off all the roaming Grunts, the two Jackals, and the single Elite (sometimes there are two) from this position. Once these enemies have been dealt with and you cannot see any other Covenant forces, inch your way up the cliffside path to your right and peek towards the left. Here, you will spot a single Grunt manning a Shade turret so be ready to kill him.

At this point, slowly scan the valley below and make sure there aren’t any red blips on your motion tracker. If you’ve successfully killed the initial group of enemies stationed in this valley without being spotted, a music cue will begin, your Marines will disperse into the valley, and the achievement will unlock.

Note: At no point should you ever drop down from the cliffside edge. If you do, the Marines are scripted to move into the valley and they will blow your cover. If this happens for any reason, simply restart the mission.