So, I've always wanted to make my own NPC's for the game. I don't mean setting a sim as an existing one, as there are plenty of tuts for that, but I mean rather creating a new NPC to do a specific job, like the NPC's in Chris Hatch's mods or Echo's carnival. I've not seen any tutorials on how to do this, but if the NPC's exist it's possible to do it somehow.
I want to make beggars and such to show up on lots in my Victorian hood, and maybe street sellers and chimney sweeps, but I don't know if it's possible to set kids as NPC's either.
IF anybody could link me to something, I'd be very grateful.

This kind of modding is pretty advanced and the people who do it have learnt to do it by trial and error and using whatever information is out there. AFAIK, there isn't a specific tutorial. Most tutorials that are available are for the straightforward stuff.

The best thing to probably do is to ask one of the people who make NPCs to take you on as an apprentice.

My suggestion would be to get involved in other general modding stuff, get comfortable with the resource structures in the game and with editing BHAVs etc., and then take a look at some existing custom NPCs and see how they're put together.