One of the things that witches in Ireland and Scotland were known for was shapeshifting, particularly into the form of hares, although some other shapes were noted in folklore as well including weasels. From a modern perspective there is a tendency to interpret these stories literally, and indeed some of them clearly indicate a literal transformation, such as we see in the 'Witch and Hare'. However many are more vague and could be interpreted as a practice of mental projection into an animal, rather than the human witch physically transforming into the shape of one*. It may also be that these stories represent a projection of the witch's spirit which would appear as an animal, rather than a literal animal. For modern witches who study traditional practices or who are interested in some of the skills attributed to historic witches this shapeshifting is certainly worth consideration.
Looking at folklore sources and witchcraft trials we might surmise this was a type of mental or trance practice, where the person learned how to project their spirit out into either an existing form or into the image of an animal (literal or figurative). Claude Lecouteux in his book 'Witches, Werewolves, and Fairies: Shapeshifters and Astral Doubles in the Middle Ages' discusses at length such a practice, wherein the witch appeared to be sleeping but the witch's spirit was wandering abroad, sometimes in human form sometimes not. During these nocturnal expeditions the witch might gather information, meet with other witches or spirits, work magic, or simply wander freely.
From a modern perspective there is no reason to think witches can't still engage in this type of trance practice, although it may be easier and easier on a person's ethics to learn how to project one's spirit out in animal form rather than learn how to take over another being's physical form. This would not be a basic or beginner skill but something to look at doing once you are fairly comfortable with the basics of sending your spirit out from your body safely (and returning safely), spiritual journeying in general, and how to handle any emergencies that might come up while in a human form. This is important because trying any of that in a new shape is going to be harder and inherently have more risk to it, and you need to be confident that if anything goes wrong you know what to do to fix it.
Looking at one of the only existing chants we have today from a historic witch, Isobel Gowdie, who claimed to deal with both the Devil and the Queen of Elfame, we see the ambiguous nature of the practice. This template also gives us a good idea of what we might want to base a structure for such a chant on as well. Some people may not see the value in this sort of thing but for a practice like this having a set ritual approach to it can add a layer of safety by providing the mind a key, if you will, to going into and out of the shape.
Isobel claimed during her trial that to go into the form of a hare she would chant:
"I shall go into a hare,
With sorrow and sych [such] and meickle [much] care;
And I shall go in the Devil's name,
Ay while I come home again."
And to return again to her own form she would say:
"Hare, hare, God send thee care.
I am in a hare's likeness now,
But I shall be in a woman's likeness even now."
This was still a dangerous practice that risked injury to the person, and the many folktales of witches who were harmed in animal form and then had the same injuries in their human forms show this. While we can, of course, interpret this as another indication of literal transformation we could also view this as a way in which the mind influences the body; the mind or spirit being injured in one form convinces the physical body it was also injured. From a metaphysical perspective this holds with the principle that what happens to us Elsewhere affects us here, while from a psychological perspective it reflects the power of the mind to influence the physical body. Many of us may be familiar with the more positive aspects of this which we see in things like placebos healing people because the people believe they will, but as with everything there is another side of that as well; what we believe hurts us can indeed hurt us.
Shapeshifting is a skill that witches have claimed and has been claimed of them for a long time. It is also something that modern witches can through different means still practice today. This blog has only looked at one potential method for witches to shapeshift, as a suggestion for those who are looking for either more advanced topics to study or new skill sets to branch into. As with so many things done by the traditional witches of the past though, this was not a safe thing and it came with no guarantees. Keep that in mind if you do decide to study this further.
*for those of you who read Terry Pratchett you'll be familiar with this concept as he writes about his witches doing it. It's what Granny Weatherwax does that they call 'borrowing'.