To unlock the Reduce, Reuse, Recycle achievement in Far Cry New Dawn, you will need to capture one of the game's 10 Outposts, and then Scavenge it from the workbench so that enemies regain control.
Once you have captured an outpost, you will gain access to a new workbench. Interact with the workbench and you will have the option to Scavenge the Outpost. Doing this will reward you with 50 Ethanol, but will also hand control of the Outpost back to your enemies and increase its difficulty level by one, up to a max level of Grade 3.
When you Scavenge a captured Outpost for the first time, you will unlock the Reduce, Reuse, Recycle achievement.