Cloning a Module's project Git repository
How to get/update a local copy of the Git repository for a contributed module, theme, or distribution
Using live previews on Drupal Core and contrib merge requests
Drupal.org core and contrib issues now have a built in live deployment preview feature, to easily allow testing of merge requests.
Merge request guidelines
General guidelines for merge requests, including scope, standards & testing
Working with patches
How to work with patches on Drupal.org.
Setting proper file permissions before committing files
File read/write/execute permissions to use for Drupal project files
Making maintainer-friendly "git am" single and multi-patches
How to make patches that include committer information and/or multiple commits
Using git blame
How to use Git's blame command to determine when a change was made
Using a Sandbox project to collaborate
How to use a Sandbox project on drupal.org as a means for a group to collaborate on a major software change