If you wish to classify every product in a SPECIFIC Source Category (that which has been pulled from your store):
Choose the top-level category to work with
Use Search Google Category to find your desired setting
Click 'Fill All Children'
Finish by clicking 'Save Changes'
If you wish to customize each Source Category (that which has been pulled from your store), you can do so just the same with this tool.
The same furniture retailer would rather classify specific groups of products with far more unique Google Product Categories.
Expand the Source Group you wish to work with
Fill each category individually
Once satisfied, click 'Save Changes'
You can continue down the list for every group you wish to fill.