What were these methods?
Until we enter Shattrath, we know only of two. The fist one was tormenting a Naaru (M'uru) in order to steal his Light powers. Kael'thas sent this M'uru to Silvermoon and ordered the creation of an order of Paladins called the Blood Knights, who unlike other Paladins didn't obtain their powers the usual way, but instead stole them from a being composed of Light energy. The second one was the use of Fel energy. Quel'thalas is magical kingdom, and through the millenia of the Sunwell's existence the Elves had come to rely on it, both personally and in their craftsmanship. All the pretty lights of Silvermoon, the hovering decorations, the tall spires and so on were sustained by magic, and without would cease to function. And collapse. With the Sunwell gone a new source of energy was required, and those were the Fel Crystals that Rommath brought with him from Outland. Their presence is also the reason why the Blood Elves have green fel-tainted eyes.
When we arrive in Shattrath we are only told what Voren'thal said to A'dal: that his visions told him that his people's only chance of survival lies with the Sha'tar. At that point we are forced to trust his visions, but once we arrive in the Netherstorm we can take a look at Kael's endeavours. He had taken over a Naaru dimensional ship and using Naaru technology created/placed Manaforges in the area. These instalations have the purpose of harvesting the raw energy found in great abundance in the Netherstorm (It being the weakest part of Outland and in the process of being torn apart by the Twisting Nether). He used this energy to gather the energy necessary to reignite the Sunwell. But in his reckless desire mishandling of them he had the Manaforges overworked, leading to malfunctions. And by 'malfunction' I mean that a mishandled Manaforge collapsed and allowed a powerful being of the Void to enter this plane of existence. That being was called Dimensius the All-Devouring, a Void Lord who utterly destroyed the Ethereal homeworld of K'aresh. Leading it's inhabitants to become the Ethereals that we are familiar with.
Also, in the Shadowmoon Valley, after we help the Earthen Ring to heal the elements of the shattered Valley, the elements tell us that the Cipher of Damnation -- the incantation used by Gul'dan to wreak such destruction -- is known to Kael'thas. And this can't be allowed to stand, as there's no telling what he will do with it. At that point the Sha'tar order Kael'thas killed and send us to the Tempest Keep.
I also forgot to mention, in the course of Sha'tar operations in the Netherstorm the Scryers discovered that Kael'thas has begun working for the Burning Legion. One more reason to bring him down.
Shortly after his demise his beaten corpse was found by Priestess Delrissa -- a Shivarra serving the Burning Legion -- who resurrected Kael by driving a Fel Crystal shard into his chest. This of course had an effect on him, and he began looking more like a Wretched than a healthy Blood Elf. Now that he had the necessary energy to reignite the Sunwell and was back on Quel'danas, he set about the task. But he did not intend to recreate the old Sunwell -- he wanted to create a gateway through which Kil'jaeden could enter Azeroth. For two reasons: Kael'thas at that point was partly power-mad, and partly insane. Along the way he also raided Silvermoon City, stealing M'uru away from the Blood Knights. This led Lady Liadrin, the Blood Knight matriarch, to renounce her vows of loyalty to House Sunstrider and join the Shattered Sun offensive -- the full might of the Sha'tar sent to thwarth Kael's schemes on Quel'danas. Kael'thas was killed shortly after.
Now, to wrap it up, I'll show why Voren'thal said that the Blood Elves would have not survived this:
Turned into demons.
Nobbel, I'll write the follow up tommorow if you really want to read my writing. I actually already started, but it's too late now and I'm tired.