Here we takes the example of the package libmpfr1ldbl, which exists in Debian Sid, but not in the stable release. We don't need to be root !
Install Debian SDK
su -c "apt-get install devscripts build-essential"
Download the .dsc file from the sid release
dget http://ftp.de.debian.org/debian/pool/main/m/mpfr/mpfr_2.4.2-3.dsc
Install dependencies as found in debian/control
cd mpfr-2.4.2 grep Build-Depend debian/control Build-Depends: debhelper (>= 7), libgmp3-dev (>= 4.2.dfsg) Build-Depends-Indep: texinfo, texlive-latex-base su -c "apt-get install debhelper libgmp3-dev texinfo texlive-latex-base"
Test if we can build successfully the package
fakeroot debian/rules binary .... .... loads of output .... ....
Build a package properly , without GPG signing the resulting binary and the resulting .changes file
dpkg-buildpackage -us -uc .... .... loads of output .... ....
Install and enjoy !
su -c "dpkg -i ../libmpfr1ldbl_2.4.2-3_amd64.deb"