Note that this is significantly easier with the stealth armor, and no specific weapons are needed as this is a No-Kill achievement, if you need to disarm traps using guns, VSV is an ideal choice. Also note that this guide includes many italicized and indented instructions as to get moral points which are not necessary to get the achievement, but do help in getting the 'enlightened' ending.
Speak with Ulman. Pick up the VSV if you don't have a silenced weapon.
Proceed to the lit hallway and wait for the guard to pass by. He may make a comment about something being wrong, but he should proceed. Note you should also take out the light by ether shooting the bulb or throwing a knife at it then retrieving it so that the soldiers to the right do not see you when you sneak there to listen.
Take the left path. You may also break right momentarily if you wish to overhear a conversation for a moral point. Note that once the guard moves away he will stand on the scaffolding in step 7 making it extremely hard to sneak past him, it is recommended to simply cut your losses with this point or go back later for it.
Hide in the alcove with the wires until the Nazi passes by.
Proceed down the hall, watching out for the tin can on the right side of the corner. Move quickly as the guard you just passed will circle back.
Move across the room over the scaffolding.
Listen to the Nazi Commandant at the bonfire sending two guards to the Generator.
At the opposite corner from which you entered this room, slip into an illuminated break in the wall and walk down the stairs. The generator is located in a room down the metal stairs near here. Shut it off.
Confirm both sides of the doorway are clear and slip into the train car in front of you. There should be one guard patrolling it.
Shoot out the lamp on the wall to the right. The guard will notice the shot and comment, but should carry on. Move through the traincar.
After jumping out of the back end of the traincar, douse the light.
For a moral point, head to the left after leaving the traincar. Scale a pile of boxes over a hole in the wall to find a secret room with some ammunition.
Proceed down the brick corridor into a large cellar area that slopes up towards a courtyard. Head to the right-most area of the room and look up the hill to see a broken floor. There will be two guards patrolling here.
Stay at the lip of the broken floor and shoot out the light directly above you. Move back slightly until you have confirmed no guards have seen you.
From the lip of the broken floor, look left. There should be a guard above you. Wait until he turns and walks away from you before making a beeline out of the broken floor to the tunnel ahead. Be aware of the guard who will be directly behind you.
Proceed down the tunnel and into a small, illuminated doorway on the left. Head down the metal stairs into a large storeroom with a light. Douse it if you wish.
Gasmask on. By the right side of the room, head through a break in the wall with some blood on the floor. This will take you into a damp trench. Proceed through it.
See dead Nazis, hear the nosalis. Proceed, there is nothing here to hurt you.
Climb up the staircase, prepare to sneak when you get up.
Slowly climb up to the ground floor. Notice the drunk Nazi.
Listen to the Nazis conversation.
Approach the man but don't enter his room or shoot out his light, instead step on the nearby glass shards and hide in a corner. He'll comment on this.
Wait a bit and he'll leave. Go blow out the light.
Carefully move into the large room nearby. Watch the guard patrols.
Listen to the Nazis talk about Metro 2, this takes a while.
Across the way, not near the camp and near the box and structural wreckage is a doorway. Move to this room and blow out the light if you'd like.
Move deeper into the tunnel and down the trap door. Equip your gas mask.
Evade the traps. After you make it back onto a concrete floor, you're home free.
Confidently proceed. Nothing is left to hurt you.
Go to Ulman. If you did not kill anyone you will get the usual confirming flash of light and the achievement/trophy will be awarded.
However, this can also be done in heavy armour as well with a bit of luck and speed. Simply sneak past the guards at the camp fire, wait for the next guard to walk past then take out the light with a throwing knife or silenced weapon. Then go to the right and right side of the next room and up a wooden poll; wait for them to finish speaking, then after he has walked past bring out your journal and book whilst sprinting wherever your compass tells you to go. You may need to take cover (such as where Charles Darwin's book is) and use a few med packs. This could be hard on higher difficulty but with enough sneaking, luck and speed you can make it.