How do you do this? Is there a tutorial on the process of plugin creation from a dll?
What sort of DLL are you talking about here? A .net DLL can just be dragged in to unity (assuming it doesn’t have any weird requirements like System.Windows). If it’s a native dll (from C++ or something), you can read about it here: Unity - Manual: Plug-ins
If you want to use Unity assets from the plugin, you’ll have to build an interface from Unity yourself. Read the info I linked for more information.
Here you can find a nice one.
https://unity3d.com/learn/tutorials/modules/beginner/live-training-archive/writing-plugins