How to make a module contribution
Until pull requests on https://git.drupalcode.org become a thing the procedure still is basically as follows:


Create an issue in that module's issue queue on drupal.org.
Git clone that module's repo and checkout the branch where you wanna fix something.
Fix the bug.
Create a patch: $ git diff &gt; [project_name]-[short-description]-[issue-number]-[comment-number].patch
Upload that patch in the issue from step 1. and set the issue to "Ready for review".


Now everybody finding the same bug can apply that patch at best using Composer and maybe cweagans/composer-patches until the patch got committed and released by one of the module's maintainers.

The full documentation can be found on drupal.org:


Making a Drupal patch with Git
Advanced patch contributor guide.