If you want to spu on someone, Astral Projection would be a better way of accomplishing this, I can't even begin to comprehend how hard it would be to actually turn yourself into a raven.
If you are dead set on doing this, it's going to take a lot of time to accomplish, and it won't be something you can just turn on and off. Something to consider: If you are not aware of what you are doing, you would become the animal you turned into, and that includes the way you think. You'd have the instincts and desires of an animal. That is, if you could even pull this one off.