Help With Some Basic Modifications to 5E

Hi all, hoping I could just tweak the current 5E ruleset with some minor changes in the code and come away with what I was hoping to do as we moved my house-ruled 5E game online using FG.

However, as a non-coder, when I dug into the tutorials I just couldn't wrap my head around how to simply copy the current 5E rules, make the tweaks, and then save as a new ruleset called "Mike's 5E" or something.

All I need to do is:

Change the modifiers for ability scores (reducing them to B/X style).
Make the default death save DC 15.
Spell save DC base is 10, not 8.

That's it!

Would any kind souls be able to assist walking me through how to tweak these rules in the core 5E rules to make my online game not a PITA?