Share a Blueprint with your Co-Op partner.
You must be in a Co-Op game in order to obtain this achievement. Go to a Workbench and to the Blueprints menu. Scroll down to your custom Blueprints (you must have created one first in Weapon Crafting). Highlight one and press to share it. Only custom Blueprints you have created will work. Anything from DLC, promotions, blueprints, or in-game rewards will not allow you to share. You do not waste resources or lose anything by sharing the Blueprints with another person in your game.