Start with checking out portage as a git submodule

$ git submodule update --init

Then update your ./portdir with

$ ./update-portdir.sh

Optionally, you can refresh the content of reduced_gen_deps.list by running:

$ ./gentoo_dep_list.py

It writes all source packages with reduced build dependencies plus the build
dependencies that can be dropped to reduced_gen_deps.list. Execution takes
about 2.5 hours.  

Create the file gen_source_pkgs.list by running:

$ ./gentoo_pkg_list_raw.sh

This file contains a list of all gentoo packages in your ./portdir

Once you have a filled reduced_gen_deps.list and gen_source_pkgs.list, check
that deb_source_pkgs.list contains all Debian source packages that you are
interested in.

The file deb2gen_mapping.list maps Debian packages to Gentoo packages. 

To inspect missing mappings from Debian source packages to Gentoo packages,
run:

$ ./find_pkg_matches.py

It will print you a list of Debian packages that miss a mapping to Gentoo
packages as well as a list of suggestions of Gentoo packages that sound
similar.

Lets summarize the data files that are present by now:

portdir               - database of ebuild files
                          * retrieved by ./update-portdir.sh
reduced_gen_deps.list - Gentoo packages with reduced dependencies
                          * generated by ./gentoo_dep_list.py
gen_source_pkgs.list  - list of Gentoo source packages
                          * generated by ./gentoo_pkg_list_raw.sh
deb_source_pkgs.list  - list of Debian source packages
                          * supplied by yourself
deb2gen_mapping.list  - mapping of Debian to Gentoo packages
                          * created manually and/or with the help of
                            ./find_pkg_matches.py

At this point, run the following script to get the list of Debian source
packages that can be mapped to Gentoo packages and also have dependencies that
can be dropped in Gentoo:

$ ./find_reduced.py

It will print:
  - a list of Debian source packages with dependencies that can be dropped
  - a list of Debian packages that cannot be mapped to Gentoo packages
  - a list of Gentoo packages that cannot be mapped to Debian packages