git submodule add https://gist.github.com/0965be641547b34618926d404773cd94.git InkscapeConverter
git submodule init
git submodule update
If you run git submodule update --remote, Git will go into your submodules and fetch and update for you.
git submodule update --remote InkscapeConverter
# get the submodule initially
git submodule add ssh://bla submodule_dir
git submodule init
# time passes, submodule upstream is updated
# and you now want to update
# change to the submodule directory
cd submodule_dir
# checkout desired branch
git checkout master
# update
git pull
# get back to your project root
cd ..
# now the submodules are in the state you want, so
git commit -am "Pulled down update to submodule_dir"
Or, if you're a busy person:
git submodule foreach git pull origin master