To connect a Planbook class to a Google Classroom class, first ensure you are connected to your Google Account via the instructions here.
When your account is connected, click on the Go To menu and select "Classes".
Here, click on a class and use the three-dot icon in the upper-right corner of the page to select "Connect Google Class"
Select the class you wish to connect in the list that appears and you'll see a Google Class entry appear on the class page.
Here, select the items you wish to integrate with your Planbook.com class.
When finished, save the changes by clicking the check mark icon in the upper-right corner of the screen.
You should now see any assignments you've created in your Google Classroom displaying in your plans on their Due Dates.
To view or edit an item, simply click on that from your plans and a new window should open with your Google Classroom account.
To enter grades for your Google items, click on the Go To menu and select "Grades".
Enter your scores in the boxes corresponding to the item and student.
Click the "Refresh Google Grades" button to receive the latest updates from your Google Account.