Step 1. Create the module folder on module/custom directory
Exemple : module/custom/test
Step 2. Create info file on the root of the module folder (module/custom/test/test.info.yml) and configure. File name must be ModuleName.info.yml
Exemple : name: Test Module
description: 'Short description'
type: module
core: 8.x
version: TheVersion
Step 3. Activate the module
On admin/modules
Or using drush en test -y