To get the Trapper trophy/achievement advance the main story of the DLC until you and Captain get drunk and wake up the next morning. After you’ve done this you will take control of Sam on top of the Captain’s building. Before you head out the Captain will ask you to arm some traps on your way down. To arm these traps you need a total of 5 Molotov Cocktails.
To arm a trap you need to have an available Molotov Cocktail in your inventory. Go up to the trap (there will be a number and X on the floor). Interact with the trap to arm it. Do this for all 5 traps and you will unlock the Trapper Trophy/Achievement. If you run out of Molotov Cocktails simply go to the room you and the Captain drank in and use the bench there to craft more. This room is accessible by going up the staircase right before the fifth trap.
Source:
https://holdtoreset.com/metro-exodus-sams-story-dlc-how-to-g...