Yesterday, I set up the campaign's goal and main antagonist. This information will give us perspective when looking at the world that the players will be interacting with.
Part 1: The Goal and The Antagonist
Part 2 - Notable Landmarks
After the goal and antagonist are clear to me, I make a note of important places and people that will help drive the story of the game. You can find your own, and there are many other spots in this adventure that deserve recognition, but for the purposes of this post, I'm sticking to these examples.
WARNING: MAJOR CURSE OF STRAHD SPOILERS
Notable Landmarks
When I look for places, I think: "What do the players take away from this place that will add to the overall narrative of the game?", "What cool moments or roleplaying opportunities can I take from here?" and "What kind of cool battles could happen if one were to break out?". With these questions in mind, I present to you the following landmarks:
The Village of Krezk
At the edge of Barovia, near the walls of mist, across from where the players start, is the fortified village of Krezk. The villagers hole themselves up in here to keep themselves safe from Strahd and his wolf minions.
Looming above Krezk is the Abbey of Saint Markovia, a madhouse overrun with wicked mongrelfolk, with echoes of the people that used to take care of the villagers. This abbey is led by a mysterious ageless figure, a man that has been taking care of it for over a century. He is known only as The Abbot. Some villagers believe this Abbot to be Strahd in disguise, but the truth is actually worse.
The Abbot it actually a corrupted Deva, sent here to carry on the legacy of a saint that died previously. Now, Strahd is such a badass, that he managed to trick the Deva to be his ally. Now The Abbot spends his time secretly helping Strahd, and he believes that whatever curse is afflicting the vampire can be dispelled if he had the perfect bride; and the fallen angel has been constructing just that. If the players go visit, they will see that accompanying The Abbot is what is effectively a Flesh Golem, meant to be delivered to Strahd as an attempt to cure him.
This is only one of the details in this village that drive home how even the most sacred of creatures can fall prey to corruption in these lands. The PCs are meant to slowly grow darker in this adventure, and this village is a good way to foreshadow or mirror this transformation in them. The Abbot is always friendly and helpful to the adventurers, at least until they start acting suspicious. An epic "prison break" type fight could break out with The Abbot letting all the Mongrelfolk loose. This place is set up to be very memorable to your players.
Yester Hill
This hill is where a bunch of evil druids that worship Strahd reside. Strahd also likes to come visit from time to time to look at some sort of visions that show him the place where he used to live or something.
This is actually, probably, the least interesting landmark in the adventure, and that's what makes it notable. This gives us a lot of space to pour our own ideas and make this part interesting. Personally, I'd place some sort of spirit that would help reveal the backstory of the land before Strahd took over; By the time the PCs get here, they would have already gotten clues about old gods and forgotten settlements that were lost because of Strahd's rule. It's a good place to show the players that these lands didn't always bend to Strahd's will.
The Amber Temple
Long ago, before Strahd became a vampire, a group of wizards built this temple in the mountains thousands of years ago to contain evil vestiges. Naturally, these evil entities eventually corrupted the wizards and made the place their own, in a way. Some time passed, and another wizard named Exethanter came in, using the knowledge passed by these vestiges to become a lich.
The Amber Temple can serve as the last dungeon before the PCs are ready to enter Castle Ravenloft, or the post-game dungeon that PCs can explore after they're done with Strahd in his castle.
If they arrive before their expedition to Castle Ravenloft, this dungeon could serve as the final way to corrupt the PCs with power, effectively turning them into monsters. It could also test the PCs mettle; How far are they willing to go in order to gain power against their enemy? Strahd could also be watching, secretly tempting them into corruption.
If the PCs arrive after killing Strahd, this could be an excellent way to tie any loose ends regarding Strahd's origins and whether he'll be able to come back. The PCs could discover that this place is tied to Strahd gaining power over the land, and they could decide to go and avoid that situation from happening again. If the PCs try to destroy stuff inside the temple, Exethanter could try to stop them. He can prove to be quite a challenge for them, and if you reveal that he was the one that helped Strahd gain his powers, it'll be all the more meaningful.
Other things to note
I also note stuff like cool or impotant NPCs and sometimes even magic items that I believe the PCs should come across. You might want to have a separate document filled with your thoughts or ideas on the contents of an adventure, and how you want to interpret them. I keep doing this until I feel I know the adventure enough to be confident to give it my own spin.