I don't think I actively taught it, it just happened once my girls were regularly walking to the car. My 3yo gets in with no dramas. My 22 month old has been able to get in herself or some time but generally doesn't. Climbing around the inside of the car like a monkey is far more appealing.
DS at two is quite capable of doing it, but won't. He's too busy doing other things so I usually have to tackle him. If he does get into the car himself he will always jump in the front and start playing with the dashboard and steering wheel.
I bruised my ribs a couple of weeks ago and it would of made life much easier if DS had climbed into his car seat for me.
I didn't actively teach them, mine figured it out themselves. My first was around the 18 month mark, he hit all of his physical milestones on the early side so don't expect yours to be able to do it at that age. Unless you have a sedan, we use a 4wd as the main family car and have some adult friends who have difficulty getting into it.
My second got the hang of it around 20 months, she's smack bang in the middle for her milestones so that's about when I'd expect most kids to manage it.
Hi OP, I found that having a stepper with our 4WD actually encouraged our eldest to try although we took her lead and didn't push the issue. By 28 months our daughter only used the stepper to climb up and did not need us to help her as before. HTH