SampleComputedIndexField.config.xml

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <configuration>
        <DefaultIndexConfiguration>
          <fields hint="raw:AddComputedIndexField">
            <field fieldName="myCustomComputedIndexField" returnType="string">
              <!-- Fully qualified assembly path to the class -->
              YourAssembly.ComputedFields.MyComputedIndexField,YourAssembly 
            </field>
          </fields>
        </DefaultIndexConfiguration>
      </configuration>
    </contentSearch>
  </sitecore>
</configuration>