Page 9 of the readme.pdf that comes packaged with the v4.x Google Checkout Module contains the exact steps:

Displaying the Google Checkout button

In order for the Google Checkout button to display, you must be using the OpenUI or another template based interface.

1. Register the module as a Third Party Module Token

OpenUI Example:
1. Expand Stores
2. Click on your Store Name link
3. Click OpenTokens
4. Click the “Add” icon
5. Specify “GOOGLE” for the module code and “GOOGLE” for the token.
6. Click “Update”

Example OpenUI Token:

%OUI%
%module|GOOGLE|checkout%

2. Place the token on the Basket page in a location according to Google Guidelines.

-Vic V.