Your wish cannot include a method you yourself don't know - It must be done in a way that you comprehend
This scene in Fate Zero describes really well how I felt about Magick in the beginning of my dealings with it. (MASSIVE SPOILER ALERT)
Kiritsugu is a hitman who only eliminates gangsters, his goal is nothing less than to achieve world peace. After a war, he acquires the wish-fulfilling Holy Grail and gets transported into it(s dream).