How do ozone molecules form in the stratosphere?

Answer 1

Ozone in the stratosphere is mostly produced from ultraviolet rays reacting with oxygen:

O2 + (radiation < 240 nm) → 2 O

O + O2 → O3

It is a two step reaction where first UV radiation spits molecular oxygen into two oxygen radicals. One or both then go on to combined with other oxygen molecules to form and ozone molecule.

Answer 2

To answer your question would require a very long answer with lots of chemical formulas. Fortunately someone has already done this and you'll find the info on Wikipediae, particularly the Chemistry of Ozone section - http://en.wikipedia.org/wiki/Ozone#Chemistry