How do I begin. I don't know how to make a texture...

[twin abyss]

I am trying to make a XRAY texture pack but I don't even know how to add a texture to the little green cube I accidentally created

[trail ridge]

💀
There should be a button that looks like 2 photos, click that, in the new menu click the little + picture button and the options there let u make your texture template

Then you should be able to paint your model