Get the source code from the NopCommerce website and open it in Visual Studio

In the plugins folder of NopCommerce solution, add a project of type class library with the plugin name prefixed by Nop.Plugin like Nop.Pugin.CategoryName.PluginName as the name. Then build the solution.

Delete the class.cs file which is added automatically to the project.

Add the references to Nop.Core.dll, Nop.Data.dll, Nop.Services.dll, Nop.Web.Framework.dll, System.Web.Mvc.dll, Autofac.dll, Autofac.Integration.Mvc.dll, EntityFramework.dll, FluentValidation.dll and others which are necessary for the plugin.

Create a text file Description.txt inside the new folder and copy the content from any other plugin and edit according to your Plugin data. This file is necessary for a plugin to work.

An example Description.txt is here.

Group: Plugin category name here
FriendlyName: Plugin Name here
SystemName: CategoryName.PluginName
Version: 1.0
SupportedVersions: 3.80
Author: Your Name
DisplayOrder: 1
FileName: Nop.Plugin.CategoryName.PluginName.dll
Description: This shows up in the Admin area as the plugin's description

Copy the web.config file from any other plugin to this plugin and don't alter anything

Change the build location from bin in plugin folder(default) to .\Presentation\Nop.Web\Plugins\CategoryName.PluginName\. This is where NoCommerce looks for plugins when loaded and the Configuration to All configurations.

Clean and build the solution