Instructions
Find another Sim you'd like to make a baby with.
Talk, flirt, kiss and hug until your relationship builds and a heart appears beneath the friendship counter.
Ask the Sim to move in or get married. This increases your household size by one.
Ask the Sim to make whoopie, and continue to do so on a regular basis until your Sim becomes pregnant.
Choose "Fertility Treatment" as a lifetime reward for one or both of your adult Sims. This increases your chances of having twins.
Buy a birthday cake and choose a small Sim to have a birthday. The Sim ages faster than normal. This may be preferable if you want a larger household but don't want multiple babies or children.