Download drupal contributed module in Drupal 8 using composer

Use the following commands to download and install in drupal 8 contributed module using Composer.

composer require drupal/{module_name} {version}

Example: composer require drupal/devel 1.0-beta1

Once execute the above command, this will download the module (not installed). For install, we need the execute the drush or drupal console command to install the module.

Via Drush: drush en {module_name}
Via Drupal console: drupal module:install {module_name}

Clear the cache in Drupal 8

We can clear the cache in Drupal 8 in many ways

1. Via CMS end
Go to Administration > Configuration > Development > Performance (/admin/config/development/performance)
Click the button "Clear all caches"

2. Drupal console
drupal cache:rebuild all

3. Drush
drush cache-rebuild / drush cr

Create custom module in Drupal 8 via Drupal console

Generate module 
 drupal generate:module - It will ask the module name and basic information.

Give the module a name of module (welcome) and Enter module description, path and basic information. Then drupal will create the below files under the your custom module.

composer.json
welcome.info.yml
welcome.module

Install module

The module now exists but it is not enabled. You don’t need to go to the module page to enable it, you can install and enable the welcome module using the module:install command:

drupal module:install welcome

This will install the module and automatically rebuild the cache.