Pt 1, Pt 2 Drag the THETA image into the Images folder of Unity.

Click on the THETA image in Unity. Go to the Texture Type in the Inspector.

Select Cubemap

Click Apply to transform into Cubemap texture.

Create a new Material in the Materials folder.

Select the new material.

In the Inspector for the material, select Shader -> Skybox -> Cubemap

Drag the material onto the sky in your scene.

Here are two additional examples using THETA images.

Next Pt 4: Unity Export to Web