If you are aware of the manner in which you're screwing this person's life up, the end result just isn't as devastating, both to you and your companion. Next, start to set your sights on what it is that you want. Remember that you're trying to hurt this person, but at the same time, do it using a method that won't be suspected. Now, choose your weapon. It can be scorn, sabotage or just plaine dishonesty.
Every occasion has an appropriate weapon.
Finally, use that implement of destruction. This isn't that hard, just do what comes naturally. Of course, you won't know what you're doing, because you don't know the true end result that you're trying to achieve. Usually, all you truly want is for your friend to be in some paine of discomfort, but material possessions are also an option, as well as your pal's other comrades. It's all up to you.