Hi, I'm creating a homebrew race that has the shell defense trait (even if I don't have the turtle race). I'm not sure exactly how to stack up all the modifiers (or actions? idk) to the shell defense, what should I do? 

I believe it’s these two if I recall correctly:

• Modifier: Set->Subtype: Minimum Base Armor->Fixed Value: 7
• Modifier: Ignore->Subtype: Unarmored Dex AC Bonus

No, I mean the one when the turtle reatreats into his shell and gains ac, falls prone, etc... 

Ahh. Please see the Subclasses & Races FAQ #2: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).