Get the Amulet of Mara
Talk to Maramal in Riften
Ask him about marriage
Wear the Amulet of Mara
Meet a marriageable NPC
Complete any required quest for that NPC
Speak to the NPC while wearing the Amulet of Mara
Choose the marriage dialogue option
Return to Maramal
Attend the wedding ceremony at the Temple of Mara in Riften
Confirm the marriage during the ceremony