First you have to fall in love with someone at free street (make sure that it's 100 social points). Then you have to invite them over to your house. Right after they say that they will come over, serve some food. Then greet them at the door and grab a plate of the food you made. your lover from free street will grab a plate too. After that, your lover will use the toilet. When they are done, do something fun with them. Finally, kiss them and propose.