You learn the strengths and weaknesses of the artist, coach them on their weaknesses and highlight their strengths. The weaknesses that they can't perfect on their own you cover up with the right guest appearances and the right instrumentation. Help them create a cohesive project, and then have them make all the rounds (radio, tv, internet etc.) Have them seen in public with the right people to vouch for whatever image it is you're trying to portray for them, plant some stories in the tabloids (charitable acts that are really planned but make them seem as though they tried to do it anonymous), and then hope the world is looking for what this artist has to offer. I've never turned an artist into a superstar so I wouldn't know what it takes but that would be, at least in part, my strategy. I'd also imagine that it takes a lot of luck and timing.