only after the last movement of the bodypart worked and for about 3-5 minutes focusing on stretching as deep as possible.
heres why you dont stretch before. You can actually tear sarcomeres which are responsible for muscles to work correctly. once these are broken down slightly, you actually can lift less weight. These sarcomeres are broken down when we lift weight naturally. No point in breaking them down prematurely, especially from deep stretching before working out. Just make sure you warm up properly and get some blood in the bodypart and you'll be fine.
BTW, there are college studies that show what i am talking about. They studied it specifically. Search PUBMED and you'll see some results