Get the Amulet of Mara
Equip the Amulet of Mara
Talk to a marriageable NPC
Complete any required quest or favor for that NPC
Return and speak to them again while wearing the Amulet of Mara
Choose the marriage dialogue option
Go to the Temple of Mara in Riften
Attend the wedding ceremony the next day
Speak to the priest to complete the marriage