Trying to understand patch management - in combination with version control

[Andreas Schildbach <andreas@schildbach.de>]

Hello everyone,

I am maintaining a rather large package and have decided to use a
version control system (SVN).

I have already svn-inject'ed the existing package into my repository.
Since the original source is (in its current form) not re-distributable
(due to licensing issues), I decided to use mergeWithUpstream-mode and
complement this with a patch system (most likely quilt).

Now, when I (or a co-maintainer) check out the project from SVN, I get
(as expected) a nearly empty project directory, containing just the
debian directory. But, how am I supposed to actually create the patches
that go into debian/patches? My current understanding is: I
modify/delete/create a set of files and use something like
interdiff/debdiff to extract the changeset. But in the case of
mergeWithUpstream-mode there is no file to modify... Don't tell me I
have to write patches "by hand" (-:

Also, can you point me to a document that explains workflow with a patch
management system in general, preferably in combination with a version
control system.

Best regards,

Andreas