Same as what purplepidjin said - take it in small steps and back off slowly.
My son is 12 and a half and we started a year and a half ago with going to the Metrocentre together, looking at maps and You Are Here boards, locating landmark shops and thinking about how to find his way. We did this a couple of times together before starting to send him off a short way ahead, with a phone to keep in contact if needed, then leading up to going round the shops on his own and meeting me later.
In time we did the same with buses (him getting on one bus and waiting for me at the other end), and now he can confidently do the journey and go shopping on his own. A couple of months ago he started going into Newcastle by himself, involving bus and Metro. He's starting a theatre group in the city centre in september and is keen to make his own way there, which is a great development for him.
Interestingly, most teens learn their independence by first doing all these things with a few friends. It seems harder for us to feel confident to allow our kids out with other teens because of the likelihood that they will be influenced negatively by the company. Ds has done his independence the other way around, on his own, and that's been necessary to give him the confidence to go out with friends.
I should add we've been lucky in that he's been able to do this during quieter times of the day because he's home educated and isn't forced to practice at busier times. I don't think he'd have coped so well (nor I!) if he'd had to find his way around the packed and noisy Metrocentre on a weekend.
Mind you, there's no way I'd let him go to the park etc to hang out with other teens because I know he wouldn't handle the banter and would be a victim, as he has in the past when he was at school. I think you have to take your lead from your child and what they feel comfortable with, and only push those boundaries when you are confident that they are ready to try.