The note to start this Prepper mission is found in the Lincoln Lookout Tower. After reading the note, find the zipline to your left (heading east). Continue using the ziplines until you reach several disconnected tree platforms. You have to leap and grab to traverse the platforms and reach the last zipline connected to the broken silo where the stash is kept. If you fall down, you’ll have to start again from the lookout. Also, kill any eagles nearby since they’ll be actively attacking you if they spot you.