Travel to Riften and go to the Temple of Mara. Inside their should be a priest called Maramal, if he's not inside the Temple of Mara then he can be found inside the Bee and the Barb giving a speech. Talk to Maramal about marriage, you don't need to buy an amulet of Mara if you already have one. Equip the Amulet of Mara and go and speak to whichever NPC you wish to marry. There will be an option asking if they are interested. Once you've selected the NPC you wish to marry head back to the Temple of Mara and speak to Maramal in order to arrange a wedding. Head outside of the temple and wait in game 24 hours and enter the temple and select "I do" during the ceremony. Once you say I do the achievement will unlock and you can speak with your wife/husband/partner and discuss where to live and they will now be found at the house you selected.