To upload avatars you will need Unity and Somnium Space SDK installed. You can follow this guide:
Tutorials and Documentation
There is few tutorials made by the community.
Upload a Ready Player Me Avatar
learn how to upload a Ready Player Me Avatar with the Avatar SDK.
Oly Sound, 26 september 2021
Upload a Mixamo Avatar
This tutorial show how you can upload any custom avatar. It use Mixamo as avatars as example, you can learn how to upload any pre-made avatar with this tutorial.
From Denevraut (22 April 2021).
Andi Rutz Complete Tutorial
How to make an avatar from scratch with Blender.
Andi Rutz, 6 April 2021
Quick Tutorial: How to upload an avatar
Quick tutorial, learn how to upload the Somnium Space example avatar.
Denevraut, 22 April 2021
Somnium Space Avatar SDK Tutorial
The official tutorial for using the Avatar SDK.
Somnium Space, 13 June 2020