First you make the NPC love you by doing expressions they like and/or giving them gifts. Next you propose by giving them a suitable ring. Once they accept your proposal they will follow you. If they stop following you then use the 'Follow Me' expression to get them to follow again. Interact with the sign in front of the home you wish to live in and select the 'Set as Marital Home' option. If you did it right a short cut-scene will ensue. Congratulations you're married.