So you know your opponent in the next round. Now what?
Check out your opponent's previous games. You could first note how they loose and how they win. Is it usually tactical shots and blows? Or is it slow grinding pawn up victories?
Then, you could think about your own style of winning and loosing and see how the two fit together. Then, you could start looking at the openings your opponent plays and match them up with yours.
A decent preparation is when you know what to play in the first 5 moves. Also that you know what type of position you are aiming for. This could be dynamic, calm, closed, open, tension in the center, no tension anywhere, castling on opposite sides, and so on. Preparing 10-15 moves ahead rarely works. Get some sleep instead.