This came up in another topic a while ago. The key elements I remember are activating the bishop, stopping white's passed pawns, and attacking the open kingside. White is going to get a dark square weakness.
Karpov played great games in this variation. Check them out for the best guidance.
The Seville Variation of the grunfeld, championed by Karpov, goes like this:
How do I play this as Black?