Here are some ideas to help with the second child / big brother or big sister transition.
When you share the baby news with your current child during pregnancy, it can be helpful to frame it as adding to your family. Let your child know how much you and your partner wanted a family, and you were so happy to have him or her. Now you and your partner are excited again to add to your family and they will be become a big sister or big brother. Express some of the complex feelings they may be experiencing: I imagine you may feel happy, sad, excited, angry, etc. Name whatever emotions they seem to be holding.
Talk with your child about the logistics of what will happen when you are in the hospital: who they will stay with; when they will come visit you (or not); when they will meet the new baby, etc.
If your older child visits you in the hospital, make sure to have some solo time with your older child in the hospital when the baby is in someone else’s arms or sleeping in the newborn crib. Ideally when they first see you, you will not beholding the baby. Have them sit or lie with you and check in together.
When you get home, you won’t be able to do everything they want you to do during your birth recovery, but you can do slow paced things each day with your older child like reading books, coloring, and cuddling.
Once you are more mobile, have special outings with just your older child. When you are out remind them that only they can eat ice cream or go on the big slide, etc., not the baby.
Tell them how much you love them and how grateful you are that they will always be your first baby, and how special it is that now they are a big sister or brother.
Use positive reinforcement for each time they do something helpful and kind with the new baby.
Expect some regression or acting behaviors. It's natural and this behavior is a young child's way of processing such a big family transition.
Continue to help name and normalize feelings. Let them know you can understand that sometimes they like being an older sister/brother and sometimes they wish they were the only child at home.
Books for kids:
Daniel Tiger: The Baby is Here
by Angela C. Santomero and Jason Fruchter
I am a Big Sister or I am a Big Brother
by Caroline Jayne Church
Books for parents:
Siblings without Rivalry
by Adele Faber and Elaine Mazlish