First, create a kid sim. Edit the personality, choose a gender, and give the sim a name. Don't do anything with the skin, head, or clothes. Now press done.
Now edit the sim. Change him/her into an adult, change the skin, body, and head to whatever you want, re-enter the name(the name is erased when you re-edit a sim), and press done. Do NOT change the personality.
Move your sim into a house or lot and check the job stats. Your sim should now have their own job stats and a job that has no pay. Just change the job to one that matches the sim's skills.