Start with acclimatisation.
Get your dog familiar with the car. The younger they are the better, but no matter how old they are, this can help. Start with a parked car, initially with doors/boot open. Get them to sit inside, have a treat, a toy or a meal and then get out. Do this is as often as you can, ideally every day. Gradually shut the doors, start the engine but don’t go anywhere. Remember every time your dog gets in the car give them a positive experience with lots of fuss and a treat or toy, whichever they prefer. How long each step takes will depend on them, it might be weeks. Start with short journeys and gradually build up the distance.
Travel sickness usually improves as they get older, but it can be a real problem for some dogs. If they are allowed to gradually become familiar with the car this often helps them to break the association between the car and the feeling of nausea. If possible avoid feeding them in the 3-4 hours before a journey. For pets that have to travel, especially long distances, your vet may be able to prescribe medication to help with the sickness.
Sometimes all this still isn’t enough, so here are some other techniques you can try. Dogs are individuals and will respond to different things, so keep trying until you see what helps your dog.