Installation

To upload avatars you will need Unity and Somnium Space SDK installed. You can follow this guide:
SDK Installation

Tutorials and Documentation

Specifications and Limitations
Optimization
Minting Guide
Animation and Gestures
Add Dynamic Bones
Test Shoulders in Blender
Test Weight Paint in Blender
Combine Materials and Textures

Avatar Tutorials

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