1. Create a folder named mymodule in the modules folder in the base directory of your Drupal site. This will be your module's directory.
2. Create a mymodule.info.yml file in your module's directory. This contains metadata that identifies the module to Drupal.
3. Add a line to the name key to provide a name for the module:

name: My Module! 

4. We will need to provide the type key to define the type of extension. We provide the module value:

type: module 

5. The description key allows you to provide extra information about your module, which will be displayed on the module's list page:

description: This is an example module from the Drupal 8 Cookbook! 

6. All modules need to define the core key in order to specify a major release compatibility:

core: 8.x 

7. Save the mymodule.info.yml file, which resembles the following code:

name: My Module! 
type: module 
description: This is an example module from the Drupal 8 Cookbook! 
core: 8.x

8. Log in to your Drupal site and go to Extend from the administrative toolbar.
9. Search for My Module to filter the list of options.
10. Check the checkbox and click on Install to enable your module: