Step 1a. Install the Acquia Cohesion modules with composer
Using composer is the preferred way of managing your modules and themes as composer handles dependencies automatically and there is less margin for error. You can find out more about composer and how to install it here: https://getcomposer.org/. It is not recommended to edit your composer.json file manually.
1. Open your terminal
Open up your terminal and navigate to your project root directory.
2. Require the module and theme in your composer.json
Now run the following commands to require the Acquia Cohesion module and Acquia Cohesion minimal theme:
composer require acquia/cohesion:6.0.3 composer require acquia/cohesion-theme:6.0.3
You'll need to change the version number above to correspond to the latest release of Acquia Cohesion which you can find on the packagist page: https://packagist.org/packages/acquia/cohesion.
Acquia Cohesion will install along with several module dependencies from drupal.org.
3. Enable Cohesion modules
You can now enable Acquia Cohesion through the UI once logged in as the admin user and navigate to: /admin/modules
Enable Acquia Cohesion core and all dependent modules together except for the "Cohesion example custom element" module as shown below:
Click the form submit button and agree to install the dependencies. You will now see a new "Acquia Cohesion" menu item in the administration toolbar.
Alternatively you can also enable the modules via drush by running the following command:
drush pm-enable cohesion cohesion_base_styles cohesion_custom_styles cohesion_elements cohesion_style_helpers cohesion_sync cohesion_templates cohesion_website_settings -y