Once you and your partner acquired the quest, talk to [Soulmate] Cupid again and he will teleport you to Love Island map. You and your partner must acquire 5 Heart Strings by killing love pigs (Blue Love Pig, Pink Love Pig, Yellow Love Pig, Red Love Pig) to finish the quest.
When proposal is accepted, talk to [King of Junon] Jorge, he will instruct Mayor Darren to facilitate your wedding.
[Mayor] Darren will walk to the chapel and you will be summoned inside the chapel for the wedding.
Inside the chapel the [Soulmate] Priest will conduct the ceremony.
You will be then spawned back to City of Junon Polis after your marriage.
TIP: Your friends may be able to watch the wedding ceremony, by telling them to talk to [King of Junon] Jorge while Mayor Darren is facilitating the ceremony.
Upgrading your Soulmate Skills
After the marriage ceremony, you and your soul mate will automatically earn 10 soulmate skill points, which you could use to purchase from [Soulmate] Cupid soulmate skill books. Only one needs to buy the skill book.
If you want to learn more soulmate skills, you and your partner must kill Dragons (Leviathan/Pythios/Tiamot) located at the Sky Dragon Isle to increase your soulmate level. Each soulmate level increase earns you 10 skill points which is equivalent to one soulmate skill. The higher your soulmate level is, the more dragons you have to kill.
For those who lost love
To divorce speak to [Mayor] Darren.
However, [Mayor] Darren is conservative and only allows divorces after 1 month of marriage.
Furthermore, the global financial crisis has arrived in City of Junon Polis and the City Council needs money, so a fee of 10,000,000z is required to divorce your Soulmate.