Instructions for reinstalling an existing Octopress on a new system, using this website (samwize.com) as an example:

git clone https://github.com/samwize/samwize.github.com samwize.com
cd samwize.com
git checkout source
gem install bundler
# Restore the themes
git submodule update --init
cd .themes/greyshade
# Ensure custom remote https://github.com/samwize/greyshade is added
git checkout -b master2 samwize/master2