How to train adult to use diapers when one is not incontinent?

vsdk, why do you want this adult to learn to use incontinence products if she or he is not incontinent? Do they have sporadic transitory incontinence? Only need the product when they are sick? Have urgency or frequency issues that create frequent "accidents"? (I assume you've discussed the basic issues with the doctor.)

If this person is still using the toilet you'll definitely want to use the pull-up kind of disposable undies, not the kind with tabs that fasten at the side. (By the way, adults don't wear diapers. They are "disposable undies," or "special panties," or something a little more dignified than diapers.)

I'm not sure what you mean by "training" the person. What part of using this product do you fear this person won't understand?

If they're not incontinent then what's the reason you want them in diapers??? If they can't make it to the bathroom in time, or if they dribble small amounts that still counts as incontinence.
You can start by calling them disposable underwear or pull ups. Take their underwear out of the drawer and replace them with incontinence products, buy the kind that are designed for active adults, they are less bulky and look and fit more like underwear. Stress that they are needed "just in case". Remind or take them to the bathroom when they get up and after meals and every couple of hours in between so they are less likely to have accidents.

If he/she is not incontinent, do not insult their dignity by insisting on diapers.