As usually...
composer create-project partageit/private-packages-bootstrap --keep-vcs
Change repositories.json to add the local repository, with local path, such as /home/... or c:/....
Build the repository:
composer build
If it can't be accessed via HTTP, copy the web folder on a Web server.
Bootstrap from your new local repository:
composer create-project partageit/private-packages-bootstrap --stability=dev --keep-vcs --repository-url=http://localhost/private-packages/