Ok....go with the assumption that I'm an idiot...because I really can be sometimes...lol. Keep this as simple and elementary as possible.
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and create their masks on their respective alpha channels

how? where?
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For a start, take a grass texture, paint the alpha channel opaque (/white), cut out a circle

Again, where? how?
I apologize sincerely, I realize I am probably frustrating you, but I really do need the instructions in a straight forward detailed manner. I would not be asking all this if there was detailed documentation. This has got to be the most poorly documented major commercial software I have ever seen.