What is it that scares off newbies (especially if they don't have a prior reason to say "By Jove, I'm going to learn Latin!")? I think it's the whole idea of declension (and the accompanying fluid word order).
No matter how many times we say, "You do this already. You already say "he, his, him" and you use 's for possession," when we slide them a pile of 5-7 cases of femina, it turns into just Crack the Code rather than a living language. On the other hand, you can't go full immersion, because it's not really a day to day language. So, stealing some ideas from Rosetta Stone, and springing from an intense debate in a seminary hallway about what "declension" actually means, I'm looking to delay learning whole paradigms of a declension, and bit by bit adding cases while getting sentences going before we learn them all.
WARNING- Don't worry about bad grammar at first. Latinglish happens.
1. Get your Latin names.
2. Call each other by them. A lot.
3. Learn sum, es, est and use with names.
4. Add pronouns. Maybe. (I'm not sure if they'll just make a mess. They are unnecessary and may tempt people to decline because English declines its pronouns.)
5. Learn sumus, estis, sunt.
6. Learn some roles and relationships for predicate nominative fodder.
7. Add those to the names, making Nom-copula-Nom sentences. (Modern kids don't learn English predicate nominative, adjective vs. objects well anymore. Take advantage of it, but make them note these are equals: Jack is a friend; the friend is Jack)
8. Add some 1&2 dec adjectives. Teach basic agreement.
9. Now, and only now, change your words. i.e. Actually decline them, break them up, chew them up Mama Bird style and spit them out. Violà, Genitive nouns. Notice the comparison to 's. I'm sticking with 1&2 dec only here.
10. More predicate nominative and predicate adjective sentences, but throw in nouns in the Genitive.
11. Substantive adjectives belong somewhere around here...
12. Now introduce the Accusative with zero explanation. Let them feel why you changed it, and notice the ubiquitous -m. (Literally, all this will be just in the singular.) This is the Rosetta-ey moment. Actually intuit the difference between an object and a subject.
13. Now talk about how we use word order to explain things. "Bobby bite puppy" ≠ "puppy bite Bobby", and "Bobby puppy bite" requires context.
14. Practice practice practice. You've got building blocks galore.
15. Introduce Dative. Get a couple objects and have people throw and give them to people while others describe it.
16. Introduce the names of all the cases and show the standard paradigm while you introduce the Ablative. Just 1st declension for a day or two, then show the 2nd declension's paradigm.
17. Now you can worry about plurals, the 3rd declension names we may have slaughtered recently (you can always give the correct 3rd Genitive along the way and just say, "It's an exception") and, of course, normal verbs.
What we've done is introduce cases by the backdoor, made people think about what language does (universal syntax), and created an intuitive link between certain letter signs and their roles (-a & -us, -am & -um, a lone vowel ending to indicate Ablative...)
Addendum: if there's one thing I'd overemphasize—both for grasping Latin cases and helping their English grammar—it's that the predicate nominatives (whether names, pronouns, roles, or substantive adjectives) stand up as equals, while we must bend, twist, and reshape our nouns to talk about doing something with those same kind of things, even though they all happen in the predicate.
I think we get killed on that in English (he/him, who/whom) because we don't teach well today that "I am the king's good servant" has no object whatsoever despite there being four words in the predicate.