2) Configure your server and run it once with Spigot/PaperMC to generate a plugins folder. You can also make the plugins directory manually. Close the server after it has created the plugins directory.
3) Download the latest public release of mcMMO from the spigot pages.
4) Drag and drop mcMMO.jar into the plugins directory
The next time you run your Spigot/PaperMC server it will start with mcMMO enabled and mcMMO will generate many configurations files for you to modify in server_directory_here\plugins\mcMMO