How to put Ubuntu environment in Qubes OS VM?

My environment is Qubes OS, but I would like to put Ubuntu in my VM to create Ubuntu environment. How can I do that?

Take a look in the documentation, there are instructions on there for doing exactly that.

Where the site?

I watched site “https://www.qubes-os.org/doc/qubes-builder/”, I finished by ‘Building Qubes from scratch’ part. But I do not know what to do from there. What specific command should I enter? (For Ubuntu)