Even when you have prepared your toddler for the arrival of the new baby, they will probably find it very hard to understand what it is all about until the baby actually arrives. So what about a bit of hands on preparation such as a bath time playset that includes a baby doll with a nappy, soap, shampoo and towel. Your toddler can change the nappy, bath the baby and wrap it up in a towel to keep it warm, just like mummy will be doing with the new baby when it arrives.
For a small child, this is a good way of introducing them to what it will be like to have a baby in the house, and to start them with the idea that they will be able to help you with the new baby. Up until now your toddler has had all your attention, so they need to get used to the idea of having another person in the house.
They need to understand that mummy and daddy will be busy with the new baby as well as juggling other commitments such as work. It can also be of great help if they learn how to hold a baby, how fragile it is and how gentle they will have to be. It is easier to avoid jealousy when the older child feels that they are your helper, rather than feeling that you are looking after another baby instead of them.