The best thing I did was tell my toddler "not yet" when he wanted me to get up to play. When you have a baby and are nursing, you can't just get up and go wherever your toddler wants you to. So when he'd ask me to go somewhere or get down on the ground to play, I'd tell him "I can't right now. Can you bring me the toy?" Or "Can you show me how to use that?" When I did eventually have the baby, he wasn't shocked that I wouldn't go wherever he wanted all the time, and most importantly, he didn't associate the baby with mom saying no. It made the transition SO much easier. Also did all the typical stuff too (big sibling book, baby doll, letting him help with baby prep, etc).
To prepare our daughter I just spoke about there being a baby in mummy’s tummy, explained having a new sibling, got some books to help understand and just dropped it into the conversation as and when without it being the sole focus! She was so prepared when we bought her new sister home and we haven’t had any change in behaviour or jealous or anything like that yet! Before going into labour… defo food prep over tidying / cleaning! I spent my 2 weeks of mat leave tidying / sorting / deep cleaning every day and then came home from hospital to a messy house because my toddler is feral 🤣 wish I’d used the time to do food prep instead, living off of takeaways and eating super later which isn’t ideal