The quest "Bolstering the Ranks" becomes available after the Dawnguard main quest "Prophet", when you return to Fort Dawnguard with Dexion Evicus. Talk to Sorine Jurard. She suggests that Florentius Baenius could be useful in the Dawnguard's crusade against the vampires, and tells you to talk to Isran to learn about Florentius' whereabouts. This begins the quest:

Bolstring the Ranks
It's been suggested that the Dawnguard could use the help of a man named Florentius, a former Priest of Arkay. I need to speak with Isran to find out if he knows where Florentius might be.

Speak with Isran

Find Isran, on the second level of Fort Dawnguard. Tell him that you need to find someone named Florentius. Isran sounds angry and asks who said something, whether it was Sorine or Gunmar. Isran does not trust Florentius and does not want him in the fort. Tell Isran that Sorine thought we'd need his help. Last Isran heard, Florentiues was aiding the Vigilants of Stendarr in Ruunvald.

Your objective is to find Florentius. Ruunvald is to the north of Riften, but if you have already been to Shor's Stone or Fort Greenwall, you can fast travel to those locations to get to your destination faster.

You will have to climb a  rocky path to the mountains. Try going around if it is too steep. You can also use your horse, Arvak, if you have it, to ease the climb. Soon you arrive at the entrance to the Ruunvald Excavation. There is a tent nearby with Volk's Journal in it. Grab the journal, then enter the excavations through the wooden door.

Note: If you find yourself in front of a locked door to Ruunvald Temple (which requires Ruunvald Key), you are not in the right place. The entrance to the Ruunvald Excavation will be to your southeast.

Enter the first room, a big chamber. Climb down the stairs to the lower level and kill the charmed vigilant that is guarding the corridor. You can loot an Amulet of Stendarr from some of these vigilants. This amulet lets you block 10% more damage with your shield. Exit the chamber north, through a narrow tunnel, then head west. Go southwest through a slightly wider corridor, and then turn south. Loot any chests and burial urns you find along the way.

Go down a wooden platform, then take a tunnel to the east. Another Charmed Vigilant, and his Husky, attack you in the next cave. Go north and descend down some stairs, deeper into the excavation.  Cross west on a wooden bridge, then through an entry where you will fight Volk. Make sure to loot Volk, for some potentially good items, and perhaps a book such as "Physicalities of Werewolves".

You will eventually arrive at a big chamber with a network of wooden platforms going around it. Go down to the lowest level, then descend a tunnel to the south and arrive at a big wooden door. Go througt the door to enter Ruunvald Temple.

Continued here: Ruunvald Temple Walkthrough