Once the round ends, the orange aura around the conversion unit and the machine by the wall will disappear. You will find three canisters coming out of the conversion unit. Collect all three of them. You need to interact with the conversion unit there times to collect each of the canisters.
Shoot the machine by the wall with your wonder weapon to get the endstation lure.
Please do not use it by accident, or else you won't be able to spawn three of them in a single round. You will be forced to go to the next round for an extra endstation lure to spawn.
If you still have Klaus, wait for him to lose his charge and return as the remote control will be in your tactical slot. The lure is acquired in the tactical slot as well.
In this next step, you need to put the three empty canisters into the essence harvesters. These essence harvesters spawn in seven different fixed locations around the map.
When you get to the harvester, you need to insert the canister in the harvester and throw your endstation lure anywhere near the harvester, and it will spawn three Tempests. Kill the Tempests to trap their soul into the canister.
The following are all the seven locations of the harvester, but they only spawn randomly in any three of the shown locations in each game.
Harvester Location #1
In the Alley next to the trash bin.
Harvester Location #2
On the Korber Rooftop next to the Speed Cola perk machine.
Harvester Location #3
On West Berline Street, next to a checkpoint office with a wall buy.
Harvester Location #4
Outside Electronics Store on East Berlin Street.
Harvester Location #5
At the Destroyed Penthouse.
Harvester Location #6
At the Ghost Station by the reception window.
Harvester Location #7
In the Sewer Access across from the Arsenal.
Find where the harvesters are in your game and interact with the harvester to insert one of the collected empty canisters.
After that, throw the endstation lure on the ground next to it, and three Tempests will spawn.
Kill all three tempests near the harvester, and the souls of these tempests will be sucked into the canister. The Tempests do not have to be super close to the harvester.
You can see the Tempest soul getting sucked into the canister in the image below. It'll be a pink-colored glowing orb flying into the harvester.
After all three Tempests have been killed, wait a couple of seconds for the canister to come out of the harvester and pick it up for delivery to the secret lab.
Remember, when you're holding the canister, you cannot sprint. You can only mantle. Do not kill any of the regular zombies on your way to the lab, and try to save as many as possible for Klaus.
You can kill special zombies such as the Manglers and Tormentors. Alot of them will spawn on this step. Killing these zombies will not contribute towards the ending of the round.
Bring the canister to the lab and insert it into the conversion unit. After inserting the first one, repeat the same process with the other two canisters and insert them into their conversion unit.
Do not forget to grab another endstation lure from the machine before leaving the lab for the second harvester.
After inserting the third and final canister, you will be asked to find a warhead.