Open Blender, and go to Edit menu / Preferences / Add Ons section. There you can use the 'Install' button at the top Right to install the Vrm for Blender Add on.
Once installed, make sure that it is turned on using the Search field to find 'VRM Import / Export'. This is the VRM for Blender addon. You can confirm it has been installed by a new tab on the main viewport labelled 'VRM'.
Install the VRM for Blender Addon
Once done you are ready to start creating your VRM Avatar in Blender.