Could you please explain how to derive the heavenly stem for a person?

First, you need to convert your date of birth into its equivalent on Chinese Lunar Calendar. Then you need to apply heavenly stems and earthly branches to the year, month, date and hour. The stems and branches are in accordance to their elements. Knowing the relationship between the elements and stems/branches, you can figure out a person's birth date easily.

It sounds easy. But the algorithm is actually complicated. I think you just need to use online tools to find out a person's birth date. It's much easier.