Back when my DC were born and maternity leave was much, much shorter, it was quite unusual for babies to go to nursery when only several months old. So my first comment is don't worry!
Yes, I expressed and used to leave as much breast milk as possible, but tins of formula as back up (I never managed to produce that much, even though I had unhurried pumping time in a lockable office every lunchtime).
Stay in close touch with the baby room staff, who should be happy to let you know everything about your DD's day when she is still new to it all (OK, if they're trying to say bye bye to several other children at the same time it might not be perfect every day, but the overall impression should be that they have plenty of time to talk to you).
Find out about things like nap schedules there. They should let her sleep when she needs to, but it's quite likely that there are set quiet/sleep times for most children and moving her routine gently to fit in will make things easier in the long run.
Send her in practical clothes that don't matter if they get stained or damaged, and which she can play easily. Expect to lose stuff! If you're leaving a bag of spare clothes, make sure things are named. Make sure you supply everything they ask for (nappies, wipes etc) and label everything.
And don't worry! It is going to be worse for you than for her (carry plenty of tissues the first days you leave her. And don't make the first day you leave her the first day you're back at work, that's way too much stress all at once).