Pretty obvious achievement: Do as it says, and deliver 50 Rucksacks back to your base.
You'll naturally unlock this as you play the game though it might take some time.
If you REALLY want this achievement for some reason and want to focus only on it, grab a large vehicle (Ambulance, the Desperado jeep, Norma, Pilato, the Rhames V pick-up and the Vagabond jeep. All of them have six space slots) and just go on back and forth supply runs. Again, this isn't recommended, as there is only so much storage space in your base, and rucksacks have a maximum capacity. it's also not the best idea to just use a bunch of cars as storage for the rucksacks.