Spend 500 chemical resources on cleaning weapons.

In Metro: Exodus, weapons degrade as they are used by becoming dirty, and this affects their performance. You can typically see the cleanliness of your weapons as the visual of them changes: if it's full of mud, it needs to be cleaned. To clean a weapon you need to get to a workbench. These can be found everywhere throughout each level. Once at one, go to your weapons, then the top option above the attachments is a bar that shows your cleanliness and you can the hold 'X' to spend chemical resources to clean it. If a weapon's cleanliness meter is empty, it will cost 10 resources to clean. Bearing this in mind, you can imagine that 500 is a lot to have to spend on cleaning them.

It's highly unlikely you will have unlocked this after a play through so the best method is to grind it out. To do this you will first need to acquire at least 30 chemical resources, find a workbench and then use your weapons until they are all dirty. Once they are all dirty, head to the workbench and make a quick save via the pause menu. Then open the workbench and clean your weapons. The dirtier they are, the more resources you can spend at a time. Once you have done this, reload your quick save and repeat cleaning them. Do this as many times as you need for the achievement. The achievement will track progress to this even after you reload your save.