How do I prepare the line for fly fishing?

The heavy casting line should need no prep. It should be clean and float well. Consider wiping a light coat of pure silicone on twenty feet of the line starting ten foot back from the tip. This helps the line slide through the fly rods line guides. Use a barbed eyelet to attach the mono filament to the casting line,, these are sold in most tackle stores (picture a short sewing needle with barbs along the shaft pointing back toward the eye). You can slide the point into the hollow casting line. Loop the mono filament through the eye and tie a clinch knot. This will give you a streamline knot that will pull easily through the water when starting a back cast.