To contribute to Seed, clone the happy_seed repo. To create a new app with the the rails generator script, run
$ export SEED_DEVELOPMENT=/path/to/your/happy_seed
$ cd /path/to/your/happy_seed
$ bundle exec happy_seed rails mytestapp
This will generate, for example, a new rails app and will set the happy_seed gem to use the source specified in SEED_DEVELOPMENT. Within that app, you can then use the regular generator commands, for example:
$ cd mytestapp
$ rails generate happy_seed:bootstrap
Since it's inconvenient to generate apps over and over, a simple pattern to test and undo your changes is
$ git commit -a
$ rails generate happy_seed:bootstrap
$ git reset --hard
Write tests where needed and submit a pull request!