This is the recommended way, using grml-live.
This example assumes you're remastering grml64 2011.12. Older versions must be remastered using release-chroots, and you must use the grml-live version matching the release (a newer version of grml-live is supposed to work as well though). For 2011.12, you must use grml-live 0.17.1 or newer. For 2012.05 consider using grml-live 0.19.3 or newer.
$ gpg --verify grml64_2011.12.iso.sha1.asc
# grml-live -A -V -u -e ~/Downloads/grml64_2011.12.iso -s testing -c DEBORPHAN,GRMLBASE,GRML_FULL,RELEASE,AMD64,IGNORE -r "Remastered" -g grml64 -o /tmp/grml64
If you have trouble using these instructions, please join our IRC support channel.
For another guide (German) take a look at Notes on remastering Grml (Thanks thkoehler).
grml-live will not work for remastering versions of Grml older than 2008. For help have a look at remastering-old wiki page.