To unlock this achievement you must customize any weapon on any operator. To do this, tab over to the Operators section from the main menu and unlock your first operator if you have not already (500 Renown). Once back to the Operators screen, select any operator that you have unlocked and then select the “Loadout” option. Then choose any customizable weapon, purchase a weapon attachment with Renown, and equip it. The attachments generally fall in the 50-250 Renown range. A good way to net Renown is to play the first Situation, CQB Basics, and obtain a three-star rating, or to just play multiplayer and/or terrorist hunt matches. Starting with nothing, it will take about 550 Renown to unlock this one. Hope this helps.
Please feel free to message me if I can make this solution any better. Otherwise, happy hunting!