There's a forge (anvil) located in the inn/pub. Simply go here and select the "Salvage" tab. To salvage a weapon hold
. Simply do this for 100 items and you will unlock the achievement. (Do note that progress isn't visually tracked).
I recommend to only salvage items you absolutely don't need. This being trinkets you already have or already have a better rarity of and weapons that are inferior to your own (and your allies) weapons. Make sure to equip the bots first with some good gear before salvaging.