This video is excellent. Other tips I would add, which worked really well for my husband, were:
Keep a couple of brightly coloured cloths in the car. Dementia affects depth perception and spatial awareness. To a pwd, the floor of the car, that you are expecting them to get into, is just a yawning black hole. They have no idea how deep it is, or how far down the floor is. Similarly, getting out of the car, tarmac can look like there's actually nothing there. So, putting a brightly coloured cloth down for them to aim at, immediately shows them where to safely put their feet. Another trick, which I had to use a few times, was to just walk my husband back in to the supermarket, and back out to the car. Eventually, it would just "click" almost like a reflex action, and he would get in the car with no problem. Walking him back into the shop and back out just broke that "stuck" thing.