Reach 8 hearts with an eligible marriage candidate
Buy a Bouquet from Pierre’s General Store
Give the Bouquet to the villager to start dating
Reach 10 hearts with that villager
Upgrade your farmhouse at least once
Repair the beach bridge and buy the Mermaid’s Pendant from the Old Mariner for 5,000g
Give the Mermaid’s Pendant to your partner
Wait three days for the wedding
Attend the wedding ceremony
Move in with your spouse after the wedding