This mission is unlocked fairly early into the game. You are required to donate 50 normal rarity (white coloured) guns to the troll booth located in Triton Flats. Every 10 guns you are required to do a little mission in regards to the weapons you are donating (retrieving them, checking on members, etc.). Since this is available rather early in the game, just keep collecting normal rarity guns enemies drop and deliver them to the drop box at your leisure. if you want to speed it up you can always buy guns from vending machines, although I'm sure your money can be put to better use. Once you donate 50 guns you get to see where your hard fought donations end up!
*NOTE*: People who are importing their save on the 360 to the One I believe are required to do this mission again, at least that was the case for me. (I completed it in normal mode and have to do it again on True Vault Hunter mode for the achievement). If someone could confirm this or mention if there is another easier way to obtain it that would be great and save some time.