Prepper Stash: D.I.Y. and D.O.A.
A prepper stash at the Doverspike Compound in John's territory, well west and south of the pumpkin farm. Note that this a relatively easy treasure to re-access, so you don't necessarily need to empty your pockets before beginning.
The stash note is on the shed out back, right where the zipline connects.
Read it to start the mission proper.
The door to the bunker is locked. You can pick it with the right perk, otherwise the key can be found inside the main house, on a dresser underneath a mounted fish.
Enter the bunker.
Down below, the stash door is unpowered.
You can follow the power lines to the switch, but it's worth nosing around the siderooms for minor lootables.
Make sure you've looted all you wanted before throwing the switch, as it will activate all the bunker's death traps, and make many areas inaccessible.
The first obstacle is a timed flame thrower. It's a simple on/off pattern, so pass through when the flame is off.
Hang a left. A chain link fence will drop down in front of you, and then fall over.
Turn right for a room with a fallen pipe and another flame jet. Keep low to pass under this one.
This will return you to the bunker door. There are sprinklers here, in case you're still smoldering a bit.
Enter the powered bunker door to complete the mission.