I'm trying to make a Sorcerer subclass, a Favored Soul variant, but am utterly new to the Homebrew coding on this site.

How do I create and add a feature that doubles Proficiency Bonus on all CHA checks and CHA skill checks? Would that be a Modifier? Would I choose a Modifier Type of "Expertise", a Modifier Subtype of "Ability Checks", and an Ability Score of "CHA"? Would I set Additional Bonus Types to "Proficiency Bonus"? Or is this not the right way to do this? It's all overwhelming!

If you did the modifier the way you described it would double the proficiency bonus for ALL ability checks. What you need to do is add several modifiers, one for each of the Charisma skills: Deception, Intimidation, Performance, and Persuasion. This is the modifier you would use:

Modifier: Expertise->Subtype: [Skill Name]

That’s it, that’s all you need. So you would add that modifier 4 times, once for each of those 4 skills I listed earlier.

And if I also want bare CHA checks to use doubled prof bonus, is there a way to do that also? Or are bare CHA checks never done?

There is nowhere on the character sheet where a plain Cha check is displayed, so there is nothing to modify. Either way, since it would essentially be expertises in all Cha checks, the player could roll a check using any of the Cha skills and it would generate the exact same bonus.