The easiest way is to go to the bazaar at night. There is a woman you can pay there to change your figure. Becoming a cowgirl at the farm also increases your bust.
You must work at the Zanti farm (the countryside is south of Tarkas, it should be easy to find). As you continue to work there, your willpower should go down. Once it reaches 85 or lower, you should trigger the next step and the path should be mostly straightforward from there.
