On Android and iOS

1. Open the app and from the main screen click on the person icon in the top right hand corner. 
2. Click on the settings link in the menu.
3. Click on Account and next to "Private Account" click the switch to turn it on. 

On the website and desktop app

1. Login to your account and click on the cog wheel on the menu in the header.
2. Click on the settings tab
3. Then under Account Settings, click the switch next to "Private Account" to turn it on.