The first thing to do to prepare for the new year, especially if your child is starting kindergarten or preschool, is to spend some time with yourself journaling or processing YOUR feelings. Yes, in order for your child to feel safe and confident, YOU have to move through any feelings you might have about them separating from you, growing up, not needing you, your unresolved feelings of your own separation from your parents, whatever. Feel them, think about them, talk about them with other caring adults AWAY from your children, write them down and have a good cry.
Good, now you’re ready. Get started on these things right away:

Drive by the school often and talk about all the great fun they are going to have learning, and playing. Be specific about the type of activities they will be doing. Walk around the school and find their classroom, the bathroom, the play area, spend some time exploring, peeking in windows and having fun. If possible visit the classroom and teacher before school starts for a little get acquainted time.
Go to the library and get some books to read about the first day of school. The librarian will be happy to help you find exactly what you need.

Start setting up school bedtimes and rituals and implementing them earlier than you need them so your child will be ready and well rested.

Decide together exactly what you will do when the first day comes, both at home getting ready and once you get to school. Practice your routine several times.
Decide together how you will say goodbye, the exact number of hugs and kisses and exactly what you will say. Practice that several times. Reassure your child that you can be counted on to do exactly as you have practiced. Stick to your word.

Find out the school’s policy on how to help crying children separate and discuss it with your child.

Discuss any concerns they might be having several days beforehand and help them find solutions to their concerns.

Reassure your child that you will be back at ____o’clock to pick them up.

If your child resists or says they don’t want to go, listen to their concerns, acknowledge their feelings and say in a kind and caring tone, “Yes I understand, and you are still going to school.” Then hum and keep moving on with your agreed upon routine.

Feel and act confident and brave in front of your child. Save any tears you have for later.

Keep your chin up, you’ll both do great!