So, the new e-books from Pottermore are out two or so days ago, and... as you can expect they're 80% texts we've already been given for free, with only two out of thirty-six being new ones. Those being the biography of Horace Slughorn and... explanation on Animagi, with a complete process of becoming one!
Now, this is a direct translation from Polish, since I haven't seen English version:
To become an Animagus, you need to be skilled in both Transfiguration and Potions. The following instruction, if followed thoroughly, shouldn't provide any negative outcomes, both physical and mental:
1. For an entire month (from one Full Moon to another), you have to hold a mandrake leaf in your mouth. You can't swallow it, or remove it. If the leaf it out, the whole process has to be started again.
2. During the second Full Moon, take the leaf out and put it (with saliva) in a small crystal vial, exposed to the moonlight (if the night is cloudy, you have to pick a new leaf and repeat Step One). To the moonlight-exposed vial, you need to add one of your hair, a (silver) spoonful of dew picked from a place untouched by sunlight or human for seven days, and a pupa of Greater Death's Head Hawkmoth. Hide the whole mixture in a quiet, dark place and don't disturb it until the next thunderstorm.
3. While waiting for the storm, every day repeat the following procedure: At sunrise and sunset touch your heart with the tip of your want and chant the words: "Amato Animo Animato Animagus".
4. Waiting for the thunderstorm may take weeks, months, years even. During this whole time, the crystal vial should be left completely undisturbed, and untouched by sunlight. Sun contamination can lead to some horrible mutations. You should also fight the temptation of opening the vial and looking at the potion, until you see lightning outside. If the sunrise-sunset incantation is done correctly, there will be a moment when - after touching your heart with your wand - you will hear an additional heartbeat, sometimes stronger and sometimes weaker then the original. If that happens, it means there's no turning back: the sunrise-sunset incantation should be repeated with no slip-ups at the right time, and never be skipped.
5. As soon as you spot the first lighting on the darkened sky, immediately go to the place where you hid the vial. If you followed the steps correctly, inside you will find a blood-red potion, enough for one gulp.
6. It is important to move to a large and secured area, where the Animagus transformation won't raise panic or won't risk your health. Touch your heart with your wand, repeat the incantation: "Amato Animo Animato Animagus" and drink the potion.
7. If everything went right, you are going to feel a burning pain and hear intense beating of two hearts. In your mind, a shape of an animal will appear. This is the animal you're gonna turn to in a moment. You must show no fear. It is too late to stop now.
8. The first transformation is usually uncomfortable and scary. Clothes and accessories (glasses, jewelry, etc.) are melting into your skin, becoming fur, scales, or spikes. You shouldn't fight or resist this change, otherwise the animal's mind may take over and do something stupid, like escape through the window, or charge at the wall.
9. When the transformation end, you should feel comfortable physically. You should remember to hide you wand beforehand in a safe spot, to easily find her when you return to your human form.
10. To transform back into human, you need to visualize yourself as one. That should be enough, but you should panic if it doesn't happen immediately. When practiced, the transformation will occur as soon as you conjure the image of the animal or human in your mind, without the need to use a wand.
Usually, witches and wizards prefer to transform with their clothes, to avoid the embarrassment of nudity, although sometimes they let them stay to pretend taking a bath. The longer the witch or wizard practices their Animagus transformation, the easier it is to choose the method of changing with or without clothes.
The Animagus' animal form is usually the same form their Patronus takes. There are no recorded cases of the Animagus form changing to adapt to a changed Patronus. Then again, there are so little wizards capable of producing a Patronus AND transforming into animals, the research lacks data.
And... I guess that's it. We have the instruction now.
