The usual way is to first get two sims to like each other to the point that both their friendship meters are at at least 50. Get both sims to lay on a bed by first clicking one side of the bed and...
First you have to make your sims really like eachother. Or if they are already married that will work too. Then all you do is click on Relax on the bed with 2 Sims, a male and female, but be sure...
The same as in any other sims 2 game. By putting 2 sims of a different sex into the bed and relaxing, then make them cuddle and try for baby. OR in the neighborood screen, hold control-shift-and c at...