How do you start new legacies?

It's hard, but I've just  done this so I've been thinking about it. It isn't enough to have an open concept ("this will be my explorer Sim for new new pack) but if I am going to settle in, I have to produce a Sim I want to play - and CAS is not my strong suit. I don't depend entirely on the randomizer - but I do need its help to get started. 

For Enchanted by Nature, my initial  concept was to take inspiration from a TS3: Supernatural  household I had played for a few years - a Fairy/Werewolf odd couple. But I had recreated them already, without their occult life-states, not long after I migrated to TS4 and before this game had those. They wound up being my first TS4 legacy founders (in 3, they had never had children). But that whole save folder was lost with a dead laptop, and I had foolishly never saved them to my library or gallery. I tool a couple stabs at re-creating them but it never took off. 

The Fairy I ended up making is a TS4 re-imagining of a Sim I had played to explore TS3: University, where he was a Rebel and an art student, who ended up doing quite well in a series of artistic careers. This one clicked, and to draw in the cross-pack play, I introduced him to Lou Howell (a Maxis premade Werewolf with good genes) and, while I am not doing babies until after university (and having more of a handle on new lifestate and skills) I am already shopping for the expansion home to move into when they need a nursery and kids' bedroom.

It clicked not so much because I was "oh, yeah - Brannon!" but because when I tackled him in CAS the randomizer fairies smiled and gave me a great base to work from, and he turned out well enough for me to want to keep playing. That was it - that's what worked. For me, anyway.

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.

Remove any mods, play vanilla. Cheat is crtl-shft-c and write testingcheats true then.... money 0

Meaning you start off with zero simoleons.

I like to start my legacy challenge by doing a rags to riches challenge with one sim that will become my legacy founder, my rags to riches is usually geared towards a specific pack or a new pack that is just released. Like I start a rags to riches every time I get a new expansion or game play pack.