The easiest way to create a plugin for XrmToolBox is to use the XrmToolBox plugin Template available on Visual Studio Gallery. You can either download and install the VSIX from Visual Studio Gallery or install the extension from your favorite IDE for .Net, Microsoft Visual Studio 2017. See this page for more information

Create your first plugin for XrmToolBox

Open Microsoft Visual Studio 2017

Click on File > New > Project

In Visual C# category, select XrmToolBox Plugin Project Template, adapt name and path for the project and click on OK

Voila! the structure of your plugin is ready. We can now focus on customizing the plugin and implementing the code logic.

Customizing your plugin

In XrmToolBox, plugins are displayed in a list on the main application form. Your plugin should have its own graphic charter so it can be visible among other plugins.

To customize your plugin graphic charter, open the file MyPlugin.cs

Update class attribute properties to define images and colors

using System.ComponentModel.Composition;
using XrmToolBox.Extensibility;
using XrmToolBox.Extensibility.Interfaces;

namespace MyXrmToolBoxPlugin1
{
    // Do not forget to update version number and author (company attribute) in AssemblyInfo.cs class
    // To generate Base64 string for Images below, you can use https://www.base64-image.de/
    [Export(typeof(IXrmToolBoxPlugin)),
        ExportMetadata("Name", "My First Plugin"),
        ExportMetadata("Description", "This is a description for my first plugin"),
        // Please specify the base64 content of a 32x32 pixels image
        ExportMetadata("SmallImageBase64", null),
        // Please specify the base64 content of a 80x80 pixels image
        ExportMetadata("BigImageBase64", null),
        ExportMetadata("BackgroundColor", "Lavender"),
        ExportMetadata("PrimaryFontColor", "Black"),
        ExportMetadata("SecondaryFontColor", "Gray")]
    public class MyPlugin : PluginBase
    {
        public override IXrmToolBoxPluginControl GetControl()
        {
            return new MyPluginControl();
        }
    }
}

• Name : the name displayed in the plugins list
• Description : A short description of your plugin purpose. Displayed in the plugins list
• SmallImageBase64 : A base64 string that represents a 32 pixels wide image
• BigImageBase64 : A base64 string that represents a 80 pixels wide image
• BackgroundColor : Background color of the plugins item in the plugins list
• PrimaryFontColor : Font color for Name, Description, Version (and Author in Large icons display mode)
• SecondaryFontColor : Font color for Author in Small icons display mode

To encode image to base64, you can use online services like https://www.base64encode.org/

Do not forget to customize also file AssemblyInfo.cs to define author and version of the plugin

using System.Reflection;
using System.Runtime.InteropServices;

[assembly: AssemblyTitle("MyXrmToolBoxPlugin11")]
[assembly: AssemblyDescription("")]
[assembly: AssemblyConfiguration("")]
[assembly: AssemblyCompany("Plugin Author")]
[assembly: AssemblyProduct("MyXrmToolBoxPlugin11")]
[assembly: AssemblyCopyright("Copyright ©  2017")]
[assembly: AssemblyTrademark("")]
[assembly: AssemblyCulture("")]
[assembly: ComVisible(false)]
[assembly: Guid("41664821-b56a-4fe9-93f5-eeada2fa8de7")]
[assembly: AssemblyVersion("1.0.0.0")]
[assembly: AssemblyFileVersion("1.0.0.0")]

Write your code logic

Your code logic is embedded in file MyPluginControl.cs. It's a UserControl that comes with a menu bar and some other features you can explore on this page.

Use third party libraries 

Sometimes, you may need to use third party libraries to provide features in your plugin. There is not restriction to do so, but you need to merge your plugin and its dependencies in one single file. This is to ensure that there won't be any version conflict between plugins using same third party Library but with different versions.

You can use ILMerge tool to merge all assemblies into a single one.