After placing the missing crests on the moondial, go down the stairs and enter Volkihar Ruins. Climb up some stairs, pull a chain, and go up yet more stairs to the east. Kill the skeletons in the big dining hall behind the door. There is a door to the south, exit the hall, after which you will be fighting a Gargoyle Sentinel. Keep exploring the castle ruins. Basically head up any stairs you find, usually in the south or the east. Come out in another big dining room with two tables in it. Pull a chain near a gargoyle statue, that causes the gargoyle to awaken. From there find the stairs leading northwest, and pull a chain to life a gate in the west.

After entering a room with a balcony, go west and through a long narrow corridor. Cross a bridge, and go up another flight of stairs. Take another door in the NW, then climb up to a huge door and go through it to enter a room with another gargoyle (Gargoyle Brute). This appears to be a dead end, but turn a candlestick in the SE corner of the room to open a secret passage. If you are still stuck, see images below to help you pinpoint the location of the candlestick:

Skyrim Dawnguard Ruined Tower Candlestick
Candlestick in dawnguard reveals secret passage

After going up some more stairs, you arrive in Valerica's Laboratory. Wait for Serana to get there (sometimes she can be slow) and then talk to her. She thinks there would be a clue in Valerica's notes. You need to locate Valerica's Journal.

Valerica's Journal Location
Go to the SW section of the lab where there are some bookshelves. Find the small red book on one of the top shelves. If you still need help finding Valerica's Journal, see the screenshots below:

Skyrim ruined tower valerica's JournalValericas Journal Location in Laboratory

Read the journal and talk to Serana. You now need to find the following ingredients for the Soul Cairn Portal:

Your objectives are:

Gather Soul Gem Shards
On a cabinet up some stairs on the side of the lab

Gather Finely Ground Bone Meal
This is on the wooden table near the portal.

Gather Purified Void Salts
Top floor of laboratory, on the shelves

Place all the ingredients on the portal vessel up in the balcony. Serana will add her blood. This will open the portal to the Soul Cairn.