Disclaimer - I'm going to be "that guy" and not search beyond the cursory work I just did to try to find the answer to this question.  (Failed my Investigation check...)

I want to add a homebrew cleric domain focused on thievery/skulking.  I see various options in the Homebrew section for other facets of the game, but nothing for homebrewing a domain.  Any pointers on what I'm missing (or a clarification stating it can't be done) would be greatly appreciated.  Thanks in advance for the assistance!

You want 'create a subclass.' Then use an existing cleric domain (Life Domain is the only free one if you don't own any content here) as a template to make sure you have all the required levels, and adjust from there.

The cleric domains are just the name of the subclass for the Cleric type so any advice you've seen for homebrewing subclasses will pertain to domains too, and also when creating this subclass if you use the option to base on an existing subclass you can use that as your baseline.

Also the Trickery Domain leans quite heavily on being good at the things you mention (going invisible, illusions so you can deceive people and not get caught, access to pass without a trace the go to stealth spell for a party, plus blessing of the trickster) so not sure what else you want out of your custom domain. Either way I'd recommend using that domain as your base to customise when you homebrew your idea.

Thanks much!  Yes, the Trickery domain is what I'm basing from, but I've found other domain suggestions outside of D&D Beyond/core rules that are more in line with what I am ultimately wanting.  I'll incorporate those using the base Trickery domain via the subclass path you've outlined.  Again, thanks for the assist!