Installation
Installing the plugin

Install the json gem <json.rubyforge.org/> on the machine where Redmine is running.
Follow the plugin installation procedure at www.redmine.org/wiki/redmine/Plugins.
Restart your Redmine.
If you already have a local Git repository set up and working from Redmine go to step 3, otherwise continue at step 2.
Adding a Git repository to a project (note, this should work whether you want to use Redmine Github Hook or not). Simplyfollow the instructions at www.redmine.org/wiki/redmine/HowTo_keep_in_sync_your_git_repository_for_redmine
Connecting Github to Redmine

Go to the repository Admin interface on Github.
Under “Service Hooks” add a new “Post-Receive URL” of the format: “[redmine_installation_url]/github_hook” (for example “example.com/github_hook”).

By default, Github Hook assumes your Github repository name is the same as the project identifier in your Redmine installation. If this is not the case, you can specify the actual Redmine project identifier in the Post-Receive URL by using the format “[redmine_installation_url]/github_hook?project_id=[identifier]” (for example “example.com/github_hook?project_id=my_project”).
That’s it. Github will now send a HTTP POST to the Redmine Github Hook plugin whenever changes are pushed to Github. The plugin then takes care of pulling the changes to the local repository and updating the Redmine database with them.