There are probably many "correct" answers to that, but I can tell you the way I discussed it with the couple of people I really talked about it with. Maybe you would feel comfortable doing this way, or maybe someone else will tell you another way you like better.

I chose to start with a pretty general, pretty short, explanation of the least complicated of things, then let them ask questions about it. This eventually led to more details, and then to the more complicated things. They, for the most part, led the conversation with their questions.

The conversation I had with one person covered everything in one sitting.

The conversation I had with the other person was broken up into smaller conversations over 2-3 of days. Mostly because I felt like breaking it up into smaller doses worked better for them.

You obviously care a great deal about this person, and I think that will help guide you in the best way to have this discussion.