Having secured your tongue of choice by way of purchase, gift or the slaughterhouse, and being confident that the same is fresh (for freshness in tongues is an important priority), proceed to wash said tongue in completely cold water, using your hands directly for preference rather than by way of gloves or other tools.
If you should consider the prospect of this contact too personally perverse, it would be acceptable to have a third-party wash your tongue for you, however, and in honest consideration of the fact that at cooking's end you'll likely be eating the object at issue, please reconsider your overall suitability to the task.
Eating tongue can conjure strange reactions in people, as can eyeballs, oxtails, the pancreas (sweetbread), testicles, and even the trotters and snouts of pigs, and it is my experience that these feelings are best examined and challenged head-on, for as with many other modalities of mental health, identity first begins within the self.
I have enjoyed tongue since first eating it at my grandmother's table, although back then I presumed that the tongue of which she talked was not the same kind of tongue she used for talking of it with (or kissing or poking or tasting the pot), and it must be said that I was never disabused by the elders for this error in my understanding.
Returning to the present and presuming all areas of ambivalence have been allayed, and with a thoroughly washed tongue now to hand, place the same into a quantity of water that has already been brought to a liberal boil and allow this boiling to continue for ten minutes, then reduce the heat and let it simmer gently until the tongue's tender.
Beef or sheep's tongue may require four to five hours, whereas a lamb's smaller tongue, being more tender from the outset, requires only three to four hours to find perfection, but when making corned tongue, you must soak the same in clean cold water over a whole night to achieve a degree of engorgement (one supposes) as required by the recipe.
Come morning put your tongue in a kettle of cold water and bring it to the boiling point, then allow it to do so for five minutes, remove the accumulated scum and simmer until tender, from three to five hours, cool just slightly in the water and then remove the skin and trim off the roots, both unpleasant jobs at best, but in the dish's service.
The beautifully varied recipes that have come down to us from all cultures have always contained wonders of flavor and taste constructed around the unwanted, less vaunted parts of flesh, for we were too poor to waste the only things we could afford in the first place and thus invention, that child of necessity, has led to meeting austerity's needs.
This is why, if one is carnivorous at least, it is a homage to eat all and waste naught, of course, in the human tide of things the tongue because of the gift of speech has both a practical and totemic enormity attached to it and a man's word is not only his bond, but his distinct self as expressed via larynx or language, but always the tongue.
Throughout the history of our species adjudged charlatans, imposters, renegades, decipherers, heretics and would-be heretics, carriers of other books, other psalms and Gods, were all liable to the severe silence of their tongue's removal (in few cases were these eaten, but recipes for the same have not, for charity, come down to us).
Serve with relish.