How do I add a Drupal 8 project (e.g. the Panels module) to my site?
Here are the steps to install from UI
Go to Manage >> Extend
Click on Install new module.
Install From URL or upload a module archive to install.
Just download module archive zip and extract to
yoursite >> modules
You can place downloaded and custom modules in modules directory.
Here in the given example installation directory is d8.
That's it. go to your module list. you can find your downloaded module there.
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