Open up your command prompt in your app/bin folder;
enter the command;
//non vendor specified cake bake plugin ContactManager //vendor specified cake bake plugin Gmcd/ContactManager
Baking the controller for your plugin;
//no vendor cake bake controller --plugin ContactManager Contacts cake bake controller --plugin Gmcd/ContactManager Contacts
Baking the Model View and Controller:
cake bake all --plugin Gerrymcdonnell/jokes jokes
Read more here:
http://book.cakephp.org/3.0/en/plugins.html#creating-your-own-plugins