Next you need to add the login details into the Magento backend at
System → Configuration → Google API → Google Base
Enter your google username. Leave off “@gmail.com” if your google username is also a gmail email address.
Enter the google password you use to log into your Google Base account.
Now you are ready to think about mapping the Magento product description fields to Google base.
This is done in the Magento backend, right here:
Catalog → Google Base → Manage attributes
Magento will allow you to create more than one attribute mapping, so there are actually two ways available for your selection at this point.
Google requires you to create a mapping for a condition. You will have to add this attribute in the attributes manager and then create a mapping for it, otherwise google will reject all products sent to it because the condition is a required attribute.
Simple Mapping Model
Create one mapping for all products. Only map the fields that are common to all products, like the price and description etc.
Categorical Mapping Model
Experiment with your mappings by creating multiple mappings that will allow you to map more fields across products that do not have the same attributes.
wigs: color and length. software: no color [|colour] or length attribute.
Product attributes and attribute sets can be created in the Magneto back end
here: Catalog → Attributes → Manage Attributes
and here: Catalog → Attributes → Manage Attribute Sets