Dd was 2.9 when ds born 9 weeks ago.
First of all, don't panic! I did, but it's fine.
Does dc1 go to childcare currently? If so, wd highly recommend keeping that on for 1-2 days if finances allow (plus they get 15 hrs/week free at 3yo). For my dd it is 'her' thing and kept some continuity in her life. And for me- well, my days alone with the baby feel like a breeze! A real sanity saver.
Make the most of the first few weeks when baby sleeps all the time, to read stories and spend quality time with dc1. Baking etc if you are up to it. This seemed to squash any jealous feelings dd was having, and was nice for me too.
Get her a 'baby' of her own- dd loves sitting with me to 'breastfeed' and talk about our babies. Yest she said 'oh, other-woman, this big baby is giving me grief! '
Make sure changing bag is packed at all times and coats/hats/shoes organised. Leaving the house can be a real flashpoint for us (sometimes we all end up crying). Or whatever is a potential flashpoint for you, get organised around that aspect of life so you don't end up frazzled.
And finally- duplo! It's the one toy dd will play with, unsupervised, for agesat a time.
Oh, also, top tip gleaned from mumsnet: when baby squeaks when you're playing w dc1, say 'just a minute [dc2] I'm busy with [dc1] right now' or similar, so dc1 doesn't feel they're the only one having to compromise on mummy time!
Enjoy it. It is hectic,but fab. Last night when dd said 'night night [baby brother], I love you', I thought, it's so worth it.