Within Fortnite, Crafting is way to create brand new Weapons, Items and Structures for your character using nothing more than the Schematics and Materials you obtain during its gameplay. The art of crafting can only take place while you are yet still on a Mission, with you needing the schematic and necessary materials for whatever you would like to construct. Once you have both of these, you can go straight into your Inventory and select the given Weapon or item you would like to build.
Once you have chosen the Weapon or item you would like to create, you will be given a list of the needed Materials you will need in order to build such a thing (as well as those given materials you presently have on hand). If you have enough material to create the weapon or item you are wanting to build, all you must do is click the Craft Button at the bottom of the screen. After the weapon or item has been crafted, it will then appear in your inventory. If you ever would like to see what materials it takes to craft something, you can view both Weapon Recipes and Trap Recipes simply by viewing the schematic or by visiting your Collection Book (viewing its Armory Section).
Unlike crafting both Weapons and items, creating structures is an entirely different process. Again, while still on a Mission of some sorts, you can easily switch over from crafting weapons to creating structures. In order to build a given structure, you will need Materials, space to build the structure, and a connection to the ground (or even another structure). From there, you can customize the appearance of your structure by selecting to either add or remove titles. You can even rotating between Wood, Metal, and Brick Structures while Building!