I don't always start a new game this way - but this is usually how I do it.
- Come up with a basic background and story line for my Sim.
- Create a young adult Sim on normal life span (no gender, trait or aspiration preference although certain traits are never picked).
- Bulldoze most if not all community lots and download one from the gallery.
- Delete pre made Sims from the neighborhoods.
- Download some 'townie' Sims from the gallery (young adults, teens and kids).
- Move Sim into a house (sometimes I build and sometimes I download the house).
- Find a career for the Sim and start building some skills (based on the story line I've created).
- Have them live there life, eventually meet a spouse and have kids.
- Have a family - normally somewhere between 1-5 kids depending on the Sims and their story line.
- Usually randomly pick the kids traits using Pinstars random trait generator. This way it is kind of like passing traits down to the kids.
- Once the kids have grown up - pick one of the kids and continue on.
I usually do this until I get bored of the family and then start a new save file. Usually play for a few generations at least. Also - sometimes I play rotationally - just depends what I feel like.