Here’s how to connect the butt section of a leader to fly line using a loop-to-loop system. You can use any kind of leader material and even nail knot a tapered leader onto the fly line and then create the loop-to-loop. I prefer to use Amnesia or Maxima Chameleon for the transition section. What diameter to choose will be determined by the thickness and rigidness or flexibilty of the fly line. Both fly line and leader material should flex together similarly.