You can find Weapon Coils and Outfit Weaves throughout the entire game. I believe the first place to acquire one is once you enter the Embrace after a couple main story missions (won't spoil anything). You usually get them from killing machines or from chests, then later on buying them from vendors or merchants. There are green, blue and purple mods you can equip to your weapons or outfits. Once you modify a weapon or outfit with one, the achievement will pop. Just so you are aware, you can not unequip the mod from that weapon or outfit until you have unlocked the perk to be able to do so, which is at the bottom of the Forager skill tree which will take about 12 skill points to unlock.