composer installcommand from inside "themag-thunder-project" directory.
cd themag-drupal-project composer install
Note that the thunder-project installs Drupal itself in the "docroot" directory (themag-thunder-project/docroot). Consider that on a production environment you have to configure your domain to point in that directory.
cd docroot ../bin/drush site:install thunder \ --existing-config \ --db-url=mysql://db_user:db_password@localhost/db_name \ --account-name="demo" \ --account-pass="demo" \ --account-mail="email@example.com" \ thunder_module_configure_form.install_modules_thunder_demo=NULL
Change db_user, db_pass, and db_name with your database info. The --account-name is your Drupal username and the --account-pass your Drupal password. The last line "thunder_module_configure_form.install_modules_thunder_demo=NULL" tells the installer to not install Thunder demo content so you can install the TheMGA Demo content later on.
../bin/drush en -y themag_thunder_demo
This articles will help you to understand how to manage your Drupal site with Composer:
Using Composer: https://www.drupal.org/docs/develop/using-composer
Drupal 8 Composer Best Practices: https://www.lullabot.com/articles/drupal-8-composer-best-practices