Obtain an Amulet of Mara
Wear the Amulet of Mara
Speak to Maramal in Riften
Ask about marriage
Complete the required favor or relationship quest for the NPC you want to marry
Return to the NPC and choose the marriage dialogue option
Wait for the wedding ceremony to be scheduled
Attend the ceremony at the Temple of Mara in Riften
Confirm the marriage during the ceremony
Move in with your spouse if desired