For most people I deal wth, especially those who are do not prefer bluntness, he sandwich method really works the best. In your email you start out by saying something hat they did well or something that went well. You then carefully transition to he bad news (whether that he they are to be fired, you couldn't get the job done, etc) and then you wrap it up with some good news, so that when they are finished reading they aren't left with a bad taste in their mouth.
However this approach doesn't work for everyone, and for some it may even prove to make matters worse between you and the recipient. For those who appreciate bluntness (you should make sure hey are this type!) sometimes breaking the bad news at the start of he conversation/email works best. However even when I do it this way, I always recommend that you finish off with something positive/constructive to lessen the blow.
As always, when discuss these sorts of matters, it is always best to be communicate face-to-face with the person, of possible, as sometimes things in email/text/phone can get misconstrued and taken out of hand. In fact meetin IRL is probably better than Skype too, as you guarantee that the person can't just abruptly stop communication and you can leverage your body communication skills to lessen the blow.
TLDR: Use the good, bad, good "sandwich" for most people, use the upfront "bad news first" method for those who like it straight and blunt. Oh and try to meet IRL to break the news.