Buy all attachments and paint jobs for one weapon.

You can do this quite early on in the game and all you need is $1200 in your wallet. Your starting gun, the 1911, only has 6 camouflages to buy worth $200 each. Once you have the money to hand, head to a shop or your Outpost locker. Select the 1911 and then press A again to 'Customise'. Now buy all 6 camouflages and the achievement will unlock. This method is one of the fasted and cheapest to do.