Nothing like the first time

This really involves no crazy effort on your part. Once you're sure that one of your companions is pregnant, just wait the required time for the pregnancy to end. This will be indicated by an egg with an exclamation point appearing above her head. Simply deliver the baby and you have a progeny and an achievement to boot. There are also several unique progeny characters that you can have join your party when you finish their side quests, which will be explained in their own achievements. Alternatively just finding an egg someplace and hatching it will also work, but where's the love, ya know?