The most straightfoward way to get a debian source package is to:

Afterwards, you may want to run dpatch apply-all in the base directory.

However, the above only works for download sources that are for your local system's specific version. If you want to look at sources for OTHER Debian versions:

  1. download package.dsc, package.diff.gz, and package.orig.tar.gz
  2. run dpkg-source -x package.dsc