1. uncomment the deb-src lines in /etc/apt/sources.list; if you do not have these, simply copy each deb line and have it begin with deb-src.

2. apt-get update

3. apt-get source <packagename>

4. cd <packagename>-<version>

5. (do modifications here)

6. dpkg-buildpackage -us -uc

7. install the .deb packages in the parent directory with dpkg -i

Do note, this does not properly embed your modifications in a patch or whatever, I haven't figured out how to do this and lose my mind at the same time.