Excel has the option to export the schema. However, there are many cases where it will not function. https://support.microsoft.com/en-us/office/export-xml-data-0b21f51b-56d6-48f0-83d9-a89637cd4360#Common%20issues%20with%20exporting%20XML%20data

I did find a workaround. If you open the excel file as an archive with 7-zip or similar tool it contains a file called xmlMaps.xml. The file contains all the xml maps for the document. Use a text editor to save each schema in a separate xsd file.

After many tutorials and manuals I've found a really useful guide about how to make the mapping of the *xls file automatically and without plug-in...very good! http://www.mrexcel.com/articles/using-xml-in-excel.php

Note: there is no solution using just Excel.

This was true in 2013 when the OP asked the question, and is still true now, as far as I can tell.

The answers provided on this page, though they use external tooling, represent the only solutions on the internet at this time, and have since been viewed 13,000 times.

If you or anyone you know have a solution using only Excel to generate XSD, please add it, because this is clearly something that would help many people.

Model the class in code:

public class Something
{
    public int ID { get; set; }
    public string First_Name { get; set; }
    public string Last_Name { get; set; }
    public string Phone_number { get; set; }
}

Save it to a .cs file and then compile it.

and then use xsd.exe to generate the xsd:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Something" nillable="true" type="Something" />
  <xs:complexType name="Something">
    <xs:sequence>
      <xs:element minOccurs="1" maxOccurs="1" name="ID" type="xs:int" />
      <xs:element minOccurs="0" maxOccurs="1" name="First_Name" type="xs:string" />
      <xs:element minOccurs="0" maxOccurs="1" name="Last_Name" type="xs:string" />
      <xs:element minOccurs="0" maxOccurs="1" name="Phone_number" type="xs:string" />
    </xs:sequence>
  </xs:complexType>
</xs:schema>