The birthdays of my daughter and her little brother are one year and nine months apart.
We bought a baby boy doll for my daughter early on. This doll became very helpful in explaining to our girl what would happen if we have another child:
It is very hard to describe the abstract concept of "another baby";
It is much easier to point to the doll and ask our girl
"what if mommy has to feed the baby and cannot hold you for a while?"
We also find it helpful to teach our girl how to "take care" of the baby doll,
such as pretending to feed the baby, changing diapers, baby proofing a playroom.
I'd like to think these activities gave her an understanding of
what we will be doing once the baby is here,
and this level of understanding eased the anxiety a little bit
whenever we have to focus on the baby.
Another thing I believe helpful was that
we treated mommy's bell as a family member
for the last few months of the pregnancy.
We will go for a walk with the "baby" (i.e., with mommy and her belly).
We will read nighttime stories with the "baby",
and we will give the "baby" a goodnight kiss when we are done.
I believe all these made the transition less jarring.