Prerequisites:
- Linux system with X GB disk space
- cdebootstrap, schroot installed
- GPG key for signing packages

1. Setting up the chroot environment for backporting

The filesystem where you create the chroot needs to be mounted with exec and
dev options, and you have to be root to do this:

  cdebootstrap -a i386 -f build etch etch32 http://debian.tu-bs.de/debian

This will create a i386 chroot with essential software for building packages in
the newly created directory "etch32". Of course, if you are backporting for a
different release, substitute its name for etch, also in the following.

2. Configuring your chroot for comfortable use with schroot

Put an entry like this into your /etc/schroot/schroot.conf (mine
is located on my external drive mounted on /exthd)

  [etch32]
  description=Debian etch i386 (etch32)
  location=/exthd/etch32
  personality=linux32
  priority=3
  users=ranke
  groups=ranke,root
  root-groups=root
  aliases=etch-ia32
  run-setup-scripts=true
  run-exec-scripts=true

You can then login into your chroot (as root) with

  schroot -c etch32

install the editor of your choice, add security and sid sources to your
/etc/apt/sources.list (in order to be able to backport from sid), add a user,
install and configure console-data for the keymap, install and configure
locales, install subversion, devscripts (for editing changelogs with dch)
and patchutils (for interdiff).

In order to build packages as a normal user, use su username to change to the
user account you want to use, then

  svn co http://kriemhild.uft.uni-bremen.de/svn/r-backports

which will set up a couple of scripts and directories that I use for backporting.
If you want to backport R, you can try

  apt-get build-dep r-base

which would pull in everything you need for building r-base, if all build
dependencies could be satisfied. This is unlikely, however, because then we
would not have to backport... Last time I just put in an etch source entry 
in my sources.list, and grabbed the build deps from there as a start.

You also need you gpg key in your build environment. I achieved this by the
following bind mount in /etc/fstab

  /home/ranke/.gnupg  /exthd/etch32/home/ranke/.gnupg     noauto bind    0       0

because I don't want my secret key on the USB drive.

3. Backporting R

Then go to directory r-base, edit the backporting script and try it with

    fakeroot sh backport_r-base.sh

You will have to install some more build dependencies that have been introduced
in newer R versions.

4. Backporting (compiling) recommended packages

This is done in the parent directory, using the script backport_recommended.sh.
You don't need fakeroot for this, because fakeroot is called in it where needed.

You need to install an R backport in order to ensure compatibility and to satisfy build
dependencies of recommended packages, which sometimes include other recommended packages
that you will need to have installed from another source.