If you have purchased your Office 365 subscription via Volume Licensing, you will need to redeem your product key in Office 365 to register as subscription licenses.
Firstly, you will need to register the license in your Licensing portal, this post assumes that this step has been completed and that you have the product key available.
Firstly, navigate to https://portal.office.com/commerce/productkeystart.aspx to start the process.
The following screenshots walk through the process. Click an image to enlarge it.