I've just started offering them in my shop. Did alot of studying beforehand and think I've come up with an easy way to do it. Read the blog post that Tiffany gave - it's really helpful.
As for how the individual can redeem the gc, I simply said that once they picked out what they wanted, they could convo me with their code, I'd make a reserved listing for them with the new price and if the cost was higher they will pay via PayPal, I'll ship and that's that. Simple is best.
Good luck!