Hello.

The chapter in our Developer´s guide you mentioned is a general, since every user wants a different functionality of his web part, so we cannot be more specific in this matter. Web part is standard user control and our documentation describes how to create it and integrate it into Kentico CMS. The particular logic itself depends on your needs. In the case you mentioned, you need to use some viewer web part onto this custom web part with transformation as its property (e.g. repeater) and then set this property to the name of requested transformation name.

Best Regards,
Radek Macalik