It's also definitely not proper grammar instruction, and even though I'm explaining the entire first declension, you're not going to see any words ending in -tive for the rest of this post, so maybe take a deep breath if that's going to bother you. I'll do a proper post sometime on why I'm committing such heresy, I promise. The short version is as follows: most of my kids aren't going to a four year college, if they go to college at all. Most of them aren't going to a college that offers Latin, if they go to college at all. I love grammar, but they don't. What they need is time in school where they are doing something that they don't hate and that stimulates their brains. I tend to lay off the grammar heavy stuff because it scares them away. If that doesn't work for you, don't do it. My students may not be your students. Feel free to take some or all or NONE of my ideas here. I'm not trying to start a revolution against grammar- just trying to get through to my own kids and share what works.
Teach them Latin using CI for a couple of months. Get them used to hearing you use nouns in different cases without making a big deal out of it. Mix in those first and second declension nouns with third declension nouns, those neuters, maybe some i-stems if you're feeling spicy. Be a big kid and even use a fifth declension dies! If you don't tell them it's hard Latin II stuff, they won't think it's hard. Really!
When enough of them have asked about "why you keep saying canis instead of canem" or whatever, it is time for the first declension unit. One day, write the following on the board. Include the English! I call this a "pattern sentence," btw.
simia piratae astronautae ariēnam in lunā dat.
The monkey of the pirate gives the astronaut a banana on the moon.
Ask them to imagine the scene. Do it as dramatically as you can pull off. Circle it: Quis dat astronautae arienam in luna? Cuius simia astronautae arienam in luna dat? cui dat simia piratae arienam in luna? Datne simia piratae astronautae arienam in VILLA? non. etc. As they get bored of it, break it up by adding details like so:
Ask them what color the monkey is. What color the moon is. Maybe draw it on the board, but encourage them to build their own mind picture with their eyes closed too. Why does that monkey give the astronaut a banana? What's the pirate's name? Get this image into their brains. Ask them to draw it themselves, if you like (they would like to). Display their drawings. They may be 17, but they still love it when mom/teacher puts their drawings on the fridge/bulletin board.