Here is the official step-by-step guide, primarily sourced from Microsoft's own documentation:
A designated KMS host computer is configured with the Office 2021 KMS host key. microsoft office 2021 ltsc professional plus 16 link
First, it's crucial to understand what "Microsoft Office 2021 LTSC Professional Plus" is and how it differs from other versions like Microsoft 365 or the standard retail Office 2021. Here is the official step-by-step guide, primarily sourced
To help you decide if you even need Office 2021 LTSC, here is a comparison: Here is the official step-by-step guide