First, open a shell and navigate to your home directory because that's where the script will reside.
Now download the git completion script using one of the following methods.
curl -O https://raw.githubusercontent.com/git/git/master/contrib/completion/git-completion.bash
In most cases, you'll want to source this file automatically whenever you start a new shell. To do so, open your ~/.bashrc file and append the command from above. Once you save the file, all new shells will automatically source the file.
Note: The tilde (~) refers to your home directory.