For the purpose of sending push notifications to cordova apps. The first step is to obtain a device token. A "device token" is specific to each device and each project.

Pre-requisite:

1. Google Cloud Messaging Project Number

For this go to Google Developer Console and create a new project.
Under Project Information is the Project Number

2. Google Cloud Messaging API Key for above Project (needed for server)

Go to Library -> Google Cloud Messaging -> Enable. Go to Credentials to create an API key of Type server.

Adding the push-plugin to project:

cordova plugin add https://github.com/phonegap/phonegap-plugin-push --variable SENDER_ID="XXXXXXX"

SENDER_ID represents the Project Id

Place the following code inside receivedEvent function within index.js

    var push = PushNotification.init({
        android: {
            senderID: "XXXXXX"
        },
        ios: {
            alert: "true",
            badge: "true",
            sound: "true"
        },
        windows: {}
    });

    push.on('registration', function(data) {
        console.log("device token: " + data.registrationId);
    });

    push.on('notification', function(data) {
           console.log(data.message);
           console.log(data.title);
           console.log(data.count);
           console.log(data.sound);
           console.log(data.image);
           console.log(data.additionalData);
    });

    push.on('error', function(e) {
           console.log(e.message)
    });

On running the above code from an Android or iOS device gives a device token.

NOTE: Device token shall be generated only on a real device not a virtual device.

For testing push notification go to this link Online Push Notification Test

For Android: Enter the Device token, Message and API key