Instructions
Have the two Sims you want to get married interact socially with each other. Select the "Chat," "Joke" and other friendly interaction options to increase the relationship between the two Sims until they become friends.
Select the "Flirt" and "Flirtatious Joke" interaction options to make the Sims flirt until they develop crushes on each other.
Select the "Kiss," "Hug," "Hold Hands" and other romantic interaction options whenever they become available.
Attend to the needs of the Sim who is going to receive the marriage proposal. She will accept it only if she is in a good mood. Make her eat, get comfortable, have fun and use the restroom to ensure she is as happy as possible.
Select the "Propose Marriage" interaction to have your Sim ask the other Sim to marry him. If the Sims' relationship meter is high enough and they are in a good mood, she will accept the marriage proposal, and the Sims will change into their formal clothing and have a small, private ceremony.