Preparations I : Before transformation is attempted.
The level of preparation will depend greatly on which technique you choose: I shall concentrate on the commoner form of magical metamorphosis, where the form changes gradually over a period of seconds or minutes.
Location: Though a simple consideration, this is often neglected. Privacy is often desirable, but it is surprising just how many people give thought to this aspect alone and ignore the more practical aspects. Ensure you have sufficient room to change: do not underestimate the size of a horse. If you are changing in a structure of some sort, be sure that as a horse you can escape it. Avoid transforming upstairs.
Physical Preparations: Unless your chosen method automatically takes care of clothing (some spells utilise it as mass), it is often advisable to undress beforehand. Many textiles, denim especially, are not sufficiently yielding to take chances with. Jewellery should be removed, footwear, and especially belts of any kind. Underwear may be retained for those with severe clothing taboos as it tends to be yielding enough to tear, but not wire-enforced garments. A towel or blanket loosely draped is probably the best solution, but if your taboo against nudity is that strong, one should consider the implications of being a horse more thoroughly.
Something many people neglect, often to their detriment, are such things as contact lenses, tooth fillings, and body piercings: make sure that where possible, such things are removed prior to transformation. A related point is the subject of horse shoes when returning to human form: best to have them removed prior to return.
Assistance: It is invaluable to have a friend or a confidant at hand both during and after the transformation. They can cover for you, provide a pair of hands in a hurry should you have overlooked something (did you unlock the back door?) and it is always useful, in today's sceptical climate, to have someone who knows and accepts that you are a horse. If you run into difficulties, they are the ones that can prevent the numerous hazards of equine life. Having an "owner" for instance, makes life as a domestic horse much more tolerable. Should you be so unfortunate as to break a leg in equine form, they are essential in preventing the traditional lamentable remedy for this state (it is worth bearing in mind that, newly come to quadrupedal life, your chances of suffering such a mishap are significantly higher than that of a natural horse).
Arrangements: Depending upon your intentions while in equine form, you should make as many provisions as possible for your equine shape before you go anywhere near that potion bottle or operating theatre. Have you allowed for food and shelter. Being pessimistic, if the worst should happen, have you made a will? Think of things such as a stash of clothing where you can get to it in case of an unexpected return to human form (along with such items as money and door keys). It is the mundane precautions that would-be equines, in their eagerness to assume their new form, so often forget, and regret.
Preparations II : During the transformation
Transformations vary greatly, and you must research the implications of the type you have chosen. Try to assume a position tenable to both human and equine postures: crouch on all fours or lie on your side: the latter tends to be preferable as almost all transformation techniques involve some loss of balance and coordination: sometimes fine motor control is lost and dizziness is a common side effect, caused by varying blood pressures and by the inevitable disorientation of inhabiting a new body. Expect wildy varying pulse rates, and an associated adrenaline rush. Though not essential, it is often a good idea to relieve oneself beforehand, as bladder and bowel control may be temporarily lost. The dramatic increase in mass often places a strain on the individual and some transformations require the subject to take on additional mass as he changes. Make sure you plan for this. Bread and cereal foods, soya beans and potatoes are all excellent bulk-building foods. Please note that some transformations can be very painful, but it is not a good idea to take pain killers before hand. Not only can these interfere with spell components, human pain killers may have an adverse condition on an equine metabolism. If you have a low tolerance to pain, avoid such methods. Though not essential, it is often a good idea to relieve oneself beforehand, as bladder and bowel control may be temporarily lost.
Preparation III : After the transformation
The greatest danger following any transformation is shock: knowing what to expect will lessen this. Every body system will have undergone major disruption, and new senses will be delivering wildly different sensory information: scent and hearing will be acutely enhanced. Taste and touch will remain largely unaffected. Eyesight can be very disorienting. Binocular vision can only be achieved by looking straight ahead, and this is recommended for the first few minutes or keeping one's eyes shut. Your eyes will no longer focus: the equine eye has a lens of fixed shape with an oval field of vision, near focused in the centre and graduated out to distant vision at the periphery. Contrary to popular belief, horses to not see in "black and white", but neither do they see colours as we do. Horses are sensitive to red and blue light, but not green: occurring in the middle of the horse's visual perception range, green objects appear white.
Feet, tail, and ears especially can cause confusing sensations: all will be emitting different signals to those received as a human. "Phantom finger syndrome" is common but quickly fades as the mind acclimatises.
In those transformations involving mental imposition, try to resist the temptation to do anything for as long as possible (at least ten minutes). New instincts will initially be almost overpowering, and the wrong stimulus at the wrong time can at best be embarrassing; at worst downright dangerous. Men should avoid transforming anywhere within scent of a mare: women should avoid stallions. The problem is made worse by one's new form often giving off a veritable "pheromone explosion" as the equine metabolism kick-starts. Any sexually receptive potential mate is liable to react with strong enthusiasm. The panic-flee response can also cause potentially dangerous reactions, given the adrenaline charged condition of the recently transformed. Any sudden noise or sudden event can cause a blind panic. Coupled with the disorientation of your new senses, a panic can cause serious or fatal injury. Be warned!
A seldom mentioned factor (the better enchantments take this into account) is the symbiotic bacteria that live in a horse's gut, without which you will be unable to digest grass. For obvious reasons, it is important to know whether or not these bacteria are present in you, and preferably not by grazing and seeing if you starve or not. If you are unsure, it is better to take precautions. Foals acquire the bacterium by eating dung, an instinctive behaviour they soon grow out of. However, a good veterinary supplier can also provide the proper culture in a (slightly) more palatable form, or subsisting on artificial feeds alone may suffice.