Go to the Microsoft link for the OpenXML SDK download. Click the red download button. On the next screen click the box next to OpenXMLSDKToolV25.msi and click next to begin the download.
Once the download is complete, launch the OpenXMLSDKToolV25.msi and follow the instructions on the screen.
The installer places the files in the following default directory:
"C:\Program Files (x86)\Open XML SDK\V2.5"
In this directory is a readme that explains how to use the SDK and a readme for the productivity tool.