Ideally you will start postnatal yoga in between 8 - 12 weeks after giving birth and most importantly get the OK from your doctor First. 
Every woman is different, every pregnancy is different, and thus every postnatal recovery period will be different. Yoga is here to help you acknowledge flexibility on the mat physically, and also adapt to our your life.
The main practice is of compassion to self. The focus is on rebalancing and restoring strength in the body.