Androgen to Estrogen

All estrogen in humans is synthesized from androgen hormones.

Androgens are a group of chemically related sex steroid hormones. Steroids are a special kind of fat molecule with a four-ringed, carbon atom backbone or core, like their cholesterol predecessor. 

A series of chemical changes turns cholesterol into androgen hormones

A series of chemical reactions spurred by proteins called enzymes remove and add groups to cholesterol's polycyclic core. These actions transform cholesterol first into the steroid pregnenolone, then into testosterone and other androgens.

As you can see from the above image, testosterone can be interconverted to estradiol, a form of estrogen. One type of cell in the ovaries creates the androgens, and another type of cell converts them into estrogen. 

Other parts of the body are also capable of converting androgens to estrogen. Fat tissue is the most significant site of androgen conversion other than the ovaries. In women, the adrenal glands normally produce small amounts of androgens, which are converted by fat tissue into estrogen. In women who are obese or who have a condition called polycystic ovary syndrome, in which the ovaries produce large amounts of androgens without converting them to estrogen, fat tissue can become a significant source of estrogen, disrupting the normal menstrual cycle and increasing the risk of endometriosis and endometrial cancer.

References:
http://www.livestrong.com/article/212632-what-causes-estrogen-in-the-body/