Instructions
Select the "Friendly" option and engage in conversations to socialize with the Sim with whom you would like your Sim to start a romantic relationship. You must build a positive relationship between the two Sims before it can develop into a romantic one.
Select "Romantic Conversation" options when the two Sims have reached a relationship level of 40 or above.
Flirt, kiss and confess attraction until the "Propose Going Steady" option appears. If the Sim rejects the proposal, select basic "Conversation" or "Amusing" options a few times before you begin the process of flirting and eventually proposing to go steady again.