For a practitioner they take the still-warm blood of an animal and douse themselves in it while naked, allowing them to feel the power while keeping themselves sane. Put the power into the parts of it you want to keep, gorge the spirit, and draw it spirit into you. This method requires an external power source.
The risks of this is that failing to exert enough will and focus will cause the power inherent in the blood to bleed over into other parts of and cause bad things to happen. They might physically change, might experience other side effects, or their mind might slip away until it fades. Opposite is that if you don’t exert enough power, you’ll get far less effect for what you have spent, but with practice you learn how much to put in, and where your attention needs to go.
Also they can use glamour alone without any other requirements or power source as it can bring whatever image they have into reality. However, this form of shapeshifting can lead to either it breaking if too fragile or becoming permanent if they sink into the glamour too deep.