There are many different ways to perform this, depending on the nature of the medium.
HydromancyEditHydromancers can see things in a water medium. The High Priestess Nimueh had this power and she used a scrying fount to magically view events in various locations. To use this power, she let a few drops of her blood (from a cut on her finger) drip into the fount and chanted a spell. It's possible that this spell (although not subtitled in the DVD) was: "Diegol cnytte, gewitte me yst, þa tacnian me yst þonne ieceþ sicle. Diegol cnytte, gewitte me yst, þa tacnian me yst þonne ieceþ sicle. Swilte ar ond calan, drædan morðor, to Camelot he cymþ" and "Diegol cnytte, gewitte me yst, aliese hine, to Camelot he cymþ". To change the images she waved her hand on the water surface. It's unknown if Nimueh could also hear noises through the scrying fount and, although she declared that she had the power to see into the future, if it could show her events that hadn't yet come to pass (The Mark of Nimueh, The Poisoned Chalice).
Before exhaling his last breath, the druid vate Lochru showed Merlin a prophecy, creating a vision in the water pool next to his death bed by simply touching the water surface. In it the young warlock was able to see the death of King Arthur by the hand of Mordred. The vision was interrupted when Arthur called his servant (Arthur's Bane: Part One).
The three soothsayers of the Disir divined using an ancient pool, which was fed by the sacred spring at Caerlanrigh. The source of the spring was a grove of yew trees in the White Mountains, called the Grove of Brineved. There, the Old Ways are at their strongest, being the Caerlanrigh the very centre of their powers (The Disir).
CrystallomancyEditCrystal-gazing (also crystal-seeing or crystallomancy) is a form of divination or scrying achieved through a trance inducted by means of gazing at a crystal. Morgause, High Priestess of the Old Religion, used a crystal to keep an eye on Arthur, Uther, and Camelot. She was probably trained by the High Priestesses in this skill and she was taught different spells to focus her powers ("Hider eft funde. On þisse ne middangeard" and "Ætíe mé þá þé ic séce") (The Sins of the Father, The Tears of Uther Pendragon). magic (which had been sucked out of him by a Gean Canach enchanted by the High Priestess) had been restored to him, showed great power being able to control one of the crystals in the Cave and scrying it. The crystal showed him at first many images of past events (from Series 1 to Series 4) and of Mordred and his Saxon army, but then the sorcerer forced it to show him where Arthur was. Merlin spoke to the King, who was sleeping with Queen Guinevere in their tent, through the crystal, warning him that Mordred was going to take them by surprise, since he had taken a hidden path in the mountains (The Diamond of the Day: Part One).
OneiromancyEditOneiromancy is a form of divination based on the interpretation of dreams. In the show, Seers have involuntary dreams in which they see the future. A seer's dreams can be stopped from not making him sleep at night if an enchanted bracelet is worn, but this only stops simple visions and not prophecies. Morgana Pendragon possesses this magical ability.
However, oneiromancy is different from the other forms of divination because Seers aren't in control of their gift. Morgana wasn't able to fully control her dreams even after she had become a High Priestess. This was shown when she had a vision of the future without expecting it The Darkest Hour: Part One.