This tutorial explains how to send and receive SMS & MMS with the Google Hangouts instead of using the standard Messaging app. A simple way to have all your Hangouts messages and text messages listed in the same screen.
Your phone must be running Android 4.0 or above, and it should also be noted that mobile carrier charges apply when you send SMS or MMS with Hangouts. To set your Google Hangouts as your default text messaging app, simply open the app and tap on the Menu button displayed at the top right corner of your screen. Go to Settings > SMS. Tick the Turn on SMS - Send and receive SMS with Hangouts checkbox:
You can also select the default SMS messaging account and configure the notification settings from the same menu.
That's it. You can now read or send SMS messages directly from Hangouts.