How do I add a project? How do I add a Drupal 8 project (e.g. the Panels module) to my site?
The third solution would be:

1- Install drush : https://www.drupal.org/node/1791676

2- Move to the local home directory of your drupal website inside the Command Prompt / Terminal (according to your OS type)

3- Type this command in the Command Prompt / Terminal (according to your OS type): 

drush en panels -y