Unlike other Hidden treasures in Far Cry: New Dawn, The Best Laid Plans focuses on puzzle elements. You need to reach the top of a destroyed bridge - its location is marked on the map below:
You need Grappling Hook perk to complete this mission. More information on "How to unlock the Grappling Hook?" page.
As always, start with reading the note. Next, you have to go towards the wagon lying on the shore. Enter inside and approach the hole leading up. Climb up.
You land on a small platform. The hook spot for your grappling hook is at the opposite end. The next step is to jump from one hook spot to another. Swing on the rope, jump and use the grappling hook on another hook spot.
You reach a larger platform with wagons' remains. Collect resources and then reach the opposite end of the platform. You find more hook spots - they will lead you to a supplies warehouse.