Consult your doctor. Check that both baby and you are doing fine, and are safe to take the trip. Most newborns can easily travel, but it's best to check with your doctor before you start planning.
Purchase window sunshades for the windows in the back of the car. This will protect your babies' eyes from the sunlight, and should help baby to sleep too. Make sure the shades fit the window properly.
Schedule the trip, so you know how long it will take. Plan in some extra time for stops, both for you to use the bathroom or get food and for changing the baby or walking around to soothe him if necessary.
Work out how many feeds you'll need for the whole journey. If you breastfeed, make sure you have access to food and drink during the journey to keep you hydrated. If you bottle feed, plan how you will make and heat the bottles. Some people use portable bottle heaters, while others will feed in service station restaurants which will often heat bottles for you.
Pack a car bag for the baby. This should include extra outfits, plenty of diapers, diaper bags and wipes, soothers and baby toys. Add some extra things for you, such as an extra top in case of accidents, snacks and a book or magazine.
Check your car seat is securely attached to your car, and well in place. If you aren't sure, you can often find trained car seat fitters in car accessory shops or baby stores.