How do I fork Ubuntu?

[Jon Kristensen]

Hello developers!

I want to apologize if this is a subject that does not belong on this
list, and I hope that I wont offend anyone by asking this question: If I
would want to fork Ubuntu, how would I go around doing that,
technically? Is there a set of files that builds Ubuntu, for example?

I want to create a small, very similar system containing most stuff but
the Ubuntu-specific and non-free packages.

Thank you!

-- 
Jon Kristensen
President of Dedikerad