I hate to be that guy but it's fairly easy to learn how to put something in someone's car on your own time. I hope your trainee has had to ever load furniture, a TV, or anything in his car or another car, otherwise then he's clueless haha. I never have gotten trained or talked to about how to load a car up. For the most part, you just do whatever the customer tells you to do and use common sense. Don't do anything questionable, be confident in what you're about to do to minimize the risk of damaging the car.
Ever try to load a grill into a little 1991 Corolla? Now THAT'S hard. I'm not even sure if it was a Corolla... it was a really little compact car from the 90's and I'm sure the guest used that car to buy a massive charcoal grill just to fuck with me and my other cart attendant. He was all "Oh can we take it out of box and we can leave the box with you guys?" and you know, then he can't ever return it, but he didn't care. He still had to make two trips to grab the fuckin' top part of the grill and a few more panels.
But like I said, your trainee should be able to use cognitive, critical, and common sense thinking skills to his advantage here. If it won't fit, don't do it, adapt to the situation, and lift what you can lift, and don't hurt yourself. I tried to get a team lift once to lift a grill into a truck bed but no one in my store even replied, just ignored me. The ladies that were there weren't much help. I was pretty peeved and I gathered the energy to just hoist it in there, my back wasn't that good after that for a few days, it kind of hurt. It helps to lift correctly, but I can barely deadlift 100lbs, and the grill was probably 300+lbs.
I didn't damage anything hardly, I must of just stretched or tore the muscle a little bit, healed in about a week + ibuprofen helps.
