Target Practice is one of the hidden treasures in Far Cry: New Dawn. This mission takes place in Nadine's Junkyard. The exact location of the mission is presented in the picture below. You can learn about this stash in Prosperity, in one of the captured outposts, or while you are exploring the world.
The place is guarded by two bandits. Eliminate them and then interact with the note. Reach the place where the sliding rope ends. You will find an opening. Go inside and then climb up the stairs. You find a large machine. The note explaining the method of opening the door is next to it. Activate the machine and then shoot the targets. The picture below shows the order in which they will activate:
Shoot all targets. After that, enter the stash to collect resources, ammo and perk points. Watch out - the exit is guarded by two bandits. They are standing with their backs turned towards you - a perfect chance for a stealth kill.