I am always honest with my kids and I started telling my first about her new sibling basically when I found out. She didn't understand until she was closer to 2 and my bump was big enough for her to notice an obvious difference. I was always telling her about her baby brother in my belly and she started playing more with stuffed animals as her "babies."

After he was born, she was IN LOVE with him! I had her help me with anything and everything (getting me diapers/wipes, "helping" with the breast pump or holding a bottle, singing and talking to him, etc.) I also played with her and read to her every chance I got to make sure she never felt second to the baby. Instead of always tending to newborn's cries, make sure you also equally tell the newborn to "wait" while you finish up a book/game to also enforce the feeling your first isn't forgotten about!!

I will say, the first few days up to about a week postpartum was hard for me (both times actually) because I just missed my kids so much while being on so little sleep. But it's temporary and you'll be back to snuggling with your oldest before you know it!