When you kill an enemy or search a totem, you may find a blueprint. This can be used to make a weapon or piece of armor.
A piece of dramatic music will play when you collect a blueprint. The music will change depending on the rarity of the item. It will sound more "epic", kind of like an intense orchestra.
The rarity of weapons, armor, runes, blueprints, etc. depends on the item's color:
Grey - Common
Red - Rarest
Press to access your equipment menu and press or to cycle through the various tabs in order to find either the weapon or armor/blueprint tab. Press to construct an item from a blueprint, providing you have the required bounty to construct one. For this achievement, you must construct 100 blueprints in this fashion.
Obviously, it's cheaper to do this with the more common items. For example, the red blueprints cost one-million bounty+ to construct, whereas the more common colours will cost far less to construct.