Reach 8 hearts with an eligible bachelor or bachelorette
Buy a Bouquet from Pierre’s General Store
Give the Bouquet to the villager to start dating
Reach 10 hearts with that villager
Repair the Beach Bridge if needed to access the Tide Pools
Buy a Mermaid’s Pendant from the Old Mariner on a rainy day for 5,000g
Make sure your farmhouse is upgraded at least once
Give the Mermaid’s Pendant to the villager
Wait three days for the wedding to happen
Attend the wedding the next morning
Keep your spouse happy by talking to them and giving gifts