You should, I guess, need to read a documentation or a manual to learn how to create an Add-in for the Excel. This is an
MSDN documentation[
^] which teaches you how to create an Add-in.
Methodology is, to create a Simple project, and right under the Visual C# look for a Office, expand it. Look for Office Add-ins and select the Excel Add-in. Visual Studio would create a project for you, and will open the source file generated for you to edit.
You can learn the Excel API from MSDN too, continue reading that document to succeed.