I want to make a weapon with this, but I don't know how to use it. I tried opening Dungeons.uproject, but all I got was an empty project.
Please add more about how to do certain things (new items, new mobs, new maps, etc.)

(ps: this is my first time with unreal engine

did you... follow all the instruction on how to extract Minecraft Dungeons?
Also, I recommend learning Unreal Engine a bit...