[Boromir88]

But back to Mansun's question: how are such manipulations carried out? What is the way to do so? Are only maiars, the first-born and selected men (Isildur and Aragon) capable of magia and goetia. If so then why only them? Did they possess some innate ability? Did their very nature that allowed them to do so?~Saurreg

With Maiar and the Elves, they just seem to possess magical capabilities naturally. It is their own innate power.

Men, it seems to be a little trickier. I don't know if Isildur has any special magical capabilities...if you are referring to the curse that he put on the Dead Army, I don't think it had anything to do with some magical ability of Isildur. Oaths are a very strong and powerful force in Middle-earth and it was more that the Dead Army were subjects of their own curse than something that Isildur did. The Men of the Mountain pledged an oath to Isildur and they went back on their oath as they fell into worshipping Sauron. Isildur just seems to serve more as an agent, as the curse was because the Men of the Mountain broke their oath made to Isildur. They weren't cursed because Isildur said they were, but because they broke an oath, and oaths have a strong binding power in Middle-earth.

Aragorn, I wonder if healing was something that he learned...we know that he lived with Elrond for a while, and he grew up with the Elves. We also know it's not just Aragorn making use of some good herbs, as he does appear to have some magical effect in combatting the Nazgul's black breath:

’Then taking two leaves, he laid them on his hands and breathed on them, and he crushed them, and straightaway a living freshness filled the room, as if the air itself awoke and tingled sparkling with joy.’~Houses of Healing

But, I wonder if he learned this from his years with the Elves, or if it was just some power and ability that he had?