Updating for A21
Here are the links you will need to get started modding A21 using Unity. You must use Unity 2021.3 to create new assets for A21.
Things you will need to follow along with the videos (note that the video talks about old version of unity but it is the same process).
Step 1: Download the Unity Hub and Unity 2021.3.19f1: https://unity3d.com/get-unity/download/archive
then Install it.
Step 2: Download my Unity 2021.3 template project: https://drive.google.com/file/d/1yPQ2yOj1Oe7pOml3ytfZcNggWWTAD4gq/view?usp=sharing then unpack it, saving it to a location you will remember.
Step 3: Open unity Hub, and click the ADD button, locate my project template, and select that folder.
You now have a properly configured modding platform. The project includes the TFP tagmanager, Linear lighting configuration, and the export script needed to export your new things as .unity3d bundles.
Step 4: Go watch the tutorial videos.
https://www.youtube.com/user/xythq/playlists?shelf_id=0&view=1&sort=dd
Step 5: Make something cool, and have fun doing it!