Method 1: Activate via Office Website (Recommended for New Installations)
This is the best method if you need to download and install Office or if direct activation within an app fails.

Visit the Office Setup Site: Go to https://setup.office.com and sign in with your Microsoft account.

Enter Your Product Key: Type your 25-character Office product key into the form and click "Next".

Link Key to Your Account: Confirm that the key has been successfully added to your Microsoft account.

Choose and Download: Select your preferred language and the 64-bit or 32-bit version (the 64-bit version is recommended for most users), then click "Redeem".

Install Office: Run the downloaded installer to install and activate Office on your PC.

Method 2: Activate Directly in an Office Application
Use this method if Office is already installed but not yet activated.

Open any Office application (like Word, Excel, or PowerPoint).

Go to File > Account (or just Account in some versions).

Click on "Activate Product" or similar option.

Enter your product key when prompted. The application will then connect to Microsoft's servers to complete activation.