The following details how to install the NextStage for Word Add-in for Microsoft 365 Commerical.
If you are trying to install the plugin on a Microsoft 365 GCC tenant, please follow these instructions instead.
Open Microsoft Word. In the Native Application, there will be a section for "Add-ins" in the top right. Click this button and select "More options". You can search for "NextStage" and then select "Add"