Answer:
first, you make your sim have a very good relationship, such as being able to make out and goose each other (kind of flirt) plus you should try making them best friends. then make two sims relax on a double bed and if the relationship is good enough there will be a "cuddle" option if you click the other sim. There should also be a "kiss", and "make out" option. make them make out a few times and there should be a "woo hoo" option. Don't press it unless you don't want them to have a baby. Next to the "woo hoo" option, there should be a " try for a baby" option unless the girl is a senior (a female adult sim can woo hoo a male senior sim, but not the other way around). Then you should have a baby. But, it doesn't always work, just like real life and so try it again to be precise ( sometimes you cannot try again on the same night and if not try the next night).