Step 1: Redeem Your Product Key
Go to setup.office.com
Sign in with the Microsoft account you want the license permanently bound to (or create a new one)
Enter your 25-character product key when prompted
Follow the on-screen instructions to associate the license with your account
Step 2: Download Office
After redeeming the key, click "Install Office"
The Office 2016 for Mac installer will begin downloading automatically
Step 3: Install Office
Open your Downloads folder and locate the Microsoft_Office_2016_Installer.pkg file
Double-click it to launch the installer
Click Continue → Agree → Install
Enter your Mac login password if prompted, then click Install Software
Wait for the installation to complete
Step 4: Activate
In the top menu bar select Finder > Applications and scroll down to the Microsoft Office apps. Open any Office app like Microsoft Word and in the What's New box that opens, select Get Started
On the Sign in to activate Office screen, select Sign in Microsoft Support
Sign in with the same Microsoft account used in Step 1
Activation happens automatically