How To Make Custom Character Models

Goodmorning everyone.

I recently started modding. I just started working on a new mod for Grand Theft Auto V. 
unfortunately I ran into some problems for another idea I had. 
my idea was to make custom characters, like the online protagonist. But is there a special way to do this? I know it’s possible through menyoo. But if I do that. I don’t have any idea how to make them spawn naturally.

I know there’s several npc mods out here, using the online protagonist model. But most of them are already set up in a way you can add them to the pedgroup list.