Download the MacOS app from the Overflow website overflow.io by clicking on the "Start for Free" button. The .dmg file will start downloading right away.
Double-click the file to install. Choose to install in the Applications folder.
You will be directed through the sign-up process. Your 30-day full-featured trial starts upon sign up.
Note: Overflow is only available for MacOS at the moment. Windows support will be coming soon.