Instructions
Decide which Sim you want to be your finance. If possible, choose a charismatic, friendly, flirty or hopeless romantic Sim to be engaged to. Due to their personality traits, you will have an easier time courting these types of Sims.
Call the Sim and invite them to your home often. Speak to her everyday to build up your friendship levels. Take her to places you are both interested in. For example, if both of you are artistic, take her to the art gallery. Visit the gym if the both of you are athletic.
Confess your attraction to the Sim or kiss him. If he rejects you, try again after increasing your friendship with each other. The Sim will become your Romantic Interest. Flirt with the Sim and continue talking to him until the option to "Propose Going Steady" shows up.
Select "Propose Going Steady" and she will become your girlfriend. Avoid flirting, kissing or hugging other Sims while you are going steady. Doing so will harm your relationship with her. Flirt and chat with your girlfriend every day. Your relationship with the Sim will continue to improve.
Choose the "Propose to Sim" option. This option will show up once your relationship bar is full. The Sim will become your Fiance after the proposal.