Marrying Serana works just like marrying any other NPC in Skyrim, so after proposing, go to the Temple of Mara to set up the wedding - and that's it. Now, when you talk to Serana, you'll have the option to propose marriage, assuming you've found and are wearing the Amulet of Mara, of course. Once Lord Harkon has been killed, talk to Valerica and tell her it is safe to return to Castle Volkihar. You'll have to complete the main quest line in Dawnguard in order for Serana to be marriable. After that, the final steps take place inside the game. Here are the steps to take if you want to marry Serana.
Fortunately, this is where fan mods come in to pick up the slack and give players what they want. While it is possible to turn your spouse into a vampire in the Dawnguard expansion, once you yourself have become a vampire, it isn't possible to marry someone who is already a vampire, like Serana. Leave the temple and wait until you receive the objective "Attend your wedding. Once done, head back to the Temple of Mara to arrange the wedding. Once you have the amulet, identify the person you would like to marry, and speak to them while wearing the amulet to propose. You can also find an amulet in random loot and shops. Maramel will sell you the amulet for gold, but there are other ways to obtain one. Maramal will instruct you to procure an Amulet of Mara.
Lord Harkon discovers a prophecy called the Tyranny of the Sun, which has led him to believe that by collecting a series of artifacts, including two elder scrolls, and sacrificing Serana, he can purge sunlight from the world, giving vampires the ability to walk freely day and night. Serana is one of the only known pureblood vampires, and as such, is titled one of the "Daughters of Coldharbour. You'll first meet Serana during the "Awakening" quest in the Dimhollow Crypt while investigating the Hall of the Vigilant attack. There are over 70 NPCs available for marriage once you have unlocked the option.