First have two sims over 100 points on the relationship chart then have one give a gift to the other and then kiss about three times then a box will pop up saying do you want to have a baby!

U have SEX with the other sim

You can either have them at 100 points, in love and in good moods.Then have repeated kisses or buy the lovematic bed and have them "whoo" :note that both ways take a while but not to hard:

U have to have a good relationship with the two opposite sex couple and have then kiss a lot of times then something will pop up!!

the man has to reles sperm