It sounds as though you are already preparing her quite nicely.
The main caution I offer is that you not talk much about the new baby until “official" preparations begin for the birth. Preschool children have only the fuzziest concept of time, and your caution that “the new baby won't be here for several months” won't mean a thing to her.
After you really begin to show that you are pregnant and she can feel movement inside your abdomen, that is a good time to begin talking about it again. Choose some time interval that she might be able to relate to in order to help her understand when the baby will arrive. “Do you remember how long it was between your birthday and Christmas? Well, that's how long we have to wait before the new baby comes.”
If any furniture adjustments are necessary (moving out of a crib, for instance, or even into a new room), make certain that is done far enough in advance to allow her to get used to the new arrangement before the baby arrives.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.