For this example we'll rebuild the GDAL Precise package.

1. Let's download the files we need from Launchpad. We need to download the .orig.tar.gz, .debian.tar.gz and .dsc.

$ cd ~/vm/ubuntugis/packages
$ mkdir gdal
$ cd gdal
$ wget https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+files/gdal_1.10.0.orig.tar.gz
$ wget https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+files/gdal_1.10.0-1~precise1.debian.tar.gz
$ wget https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+files/gdal_1.10.0-1~precise1.dsc

2. With these files downloaded, we simply need to extract the .dsc

$ dpkg-source -x gdal_1.10.0-1~precise1.dsc gdal

3. Then you can cd into the folder and update the changelog for a rebuild or make change to the package if you want.

dch -p -D precise -l precise "Launchpad rebuild for Precise"

You can then open debian/changelog and add entries if you have made other changes to the package.