What is the molecular mechanism of cystine bond formation?
If your question is with respect to a eukaryotic cell, the di-sulfide bridge/bond is formed in the rough endoplasmic reticulum which is an oxidative environment (unlike most other organelles which are reductive). This paper may be of relevance to you:  

Pathways for protein disulphide bond formation - Frand et al, Trends Cell Biol. 2000 May;10(5):203-10.