It takes a bit of doing to get a self sustaining population. Basically you need a critical mass of people, plus an abundance of housing. Your population will resemble a sine wave since you have periods of die offs and births due to old people not leaving their houses voluntarily for young people.
You also need to make sure that the low peak of your population is always enough to staff your food producing facilities, and the best way to produce the most food with the fewest workers is a firewood trading economy.
Then, just have a bunch of barns for all the stuff to get stored, and you've got a self sustaining town.