Step 1: Of course import whatever avatar you want to do this on, to Unity (With SDK and Poyoimi, if you don't know what these are you probably should not be here)
Step 2: Import your png by dropping it in the assets
Step 3: Right click on your hierarchy and create a new sphere object (under 3D Objects), place it right beneath your avatar in the hierarchy
Step 4: Make the sphere relatively big (using the scale numbers) so it surrounds you: (10, 10, 10) is good
Step 5: Create a new material in your assets and apply it to the sphere in your scene (drag and drop)
Step 6: Then apply whatever png file you wanted to the sphere as well now (drag and drop)
Step 7: Now you need to get the shader so go to VRCmods and download the rotating panosphere shader https://vrcmods.com/item/626 (then import it to your assets)
Step 8: Change the shader on your sphere/material to the new one you just imported (panosphere v2)
Step 9: Test to see if it works by pressing play (adjust camera if needed)
Step 10: If the speed of the rotation is too high, you can just adjust by bringing the X and Y axis sliders closer to the middle
Step 11: (Optional) If you want, you can treat the sphere as an object that you may want to spawn and despawn on command but you are gonna have to go to a different tutorial to see how to add a toggle for that; for now the rotating sphere will be constantly on
Step 12: Upload the avatar using the Control Panel in the VRChat SDK Menu