How do you start new legacies?

I generally do a legacy challenge. Over the generations, this then turns into a dynasty spread out across multiple households. I'm currently playing in a save with three major competing/overlapping dynasties from three different challenges. After finishing the challenge, I'll often use offshoots of the dynasty for spin-offs. 

In picking a founders personality, I might randomize them, or I might pick out traits that will help them with their challenge, or I'll use the MyStory tool to develop them. One thing I like to do is pick a fictional character I like and answer the MyStory quiz as though I was that character.

For genetics, since I'm not much into CAS, I'll usually roll the randomizer until I get someone I'm pretty satisfied with. Then I'll tweak them a little, probably using presets. For clothes, I'll pick out their everyday look with care, usually drawing inspiration from their personality, unless I'm playing a challenge that comes with a generational dress code. Then I skim over the rest of their outfits to see if any of them will land my founder on the Townie Fashion Police most wanted list. If they do, I'll do a few tweaks to make them less egregious. I'll usually go back and change those outfits later on, when I get to know the character better.