Instructions
Achieve a 100-point relationship score between the two Sims. Achieving a 100-point relationship score results in the two Sims falling in love. To get a high score, have the two Sims talk to one another, tell jokes, hug, kiss and hang out. To check your relationship score status, click on the Relationship tab in the menu bar and click the picture of the Sim you're getting into a relationship with.
Go to your Crafting Table located on the first island and create two Marriage Beanies, one for each Sim. Click on the crafting table. Click on "Make Marriage Beanies." The Sim will take over from there.
Craft two Aloha Wedding Leis, one for each Sim. Click on the Crafting Table. Click on "Make Aloha Wedding Leis." The Sim will take over.
Put the beanies and the leis on each Sim by clicking on each item from each Sim's inventory.
Allow one Sim to walk up to the other by clicking on a spot near the other Sim. Once they are close, click on the other Sim and click "Romantic." Click "Propose" from the new list of menu options.
Watch one Sim propose to the other. As long as they have a 100-point relationship score, the Sim will accept the proposal.