# Make sure you are in the repo directory
cd <somewhere>

# Add the external repository as a submodule
git submodule add <address_of_submodule_repo>

# Make sure to initialize it and update (when required)
git submodule init
git submodule update

# Check status, add everything (if needed), then commit
git status
git add .
git commit -m "Added <submodule/library> as a git submodule."