The following is what I usually do when I want to debianize a Haskell stuff from Hackage. This is a personal recipe, by no means official procedure or such. I may be wrong, badly at that.
Check pkg-haskell repository whether your intended package already exists or not. Somebody might work on it already.
Find the webpage of your intended package at HackageDB, then download "Cabal source package". Untar it. The tarball filename should be changed to fit the Debian source package convention. For example, if the original name is something like foobar-1.0.tar.gz, then it should be haskell-foobar_1.0.orig.tar.gz.
File ITP. reportbug is your friend.
Run cabal-debian in untared dir:
$ cabal-debian --debianize --quilt
We do have the upstream tarball, so use --quilt. See http://wiki.debian.org/UsingQuilt. It might give you warnings about changelog. debian/changelog doesn't exist yet, so ignore it.
carefully. Then do things as they say. These two are short, but well-written instructions.
Edit the generated debian/control appropriately. Do not forget to make your debian/copyright machine-friendly. See http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/.
Test whether your package is flawless and really buildable. Do lintian. Use pbuilder.
$ lintian haskell-foobar_1.0-1_amd64.changes # pbuilder --build haskell-foobar_1.0-1.dsc
Darcs is quite easy to use, and you don't have to be the master of Darcs to merely debianize things anyway. Basically, what I usually do is
$ darcs record -a; darcs push --repo=debian
Darcs is well-suited to Haskell-related development, but seems you can use Git, too.
Change debian/changelog. "UNRELEASED" in the first line should be "unstable" or "experimental". Do not forget to close ITP.
$ debuild -tc; debcommit -r; debrelease; darcs push --repo=debian
That's all folks!