Tying fly line to a leader is EZ, with the EZ knot, this makes a nearly invisible knot, one that does not splash.  Because of the way the knot bites into the fly line, the knots over all diamiter, is smaller than the fly line alone.  You First tie the standard EZ knot on the tool, then you slide the fly line under the loops as shown, carefully slide the knot of the tool, and carefully work the loops togeather as you tighten the knot.  The knot will hold fine even if it is not a neat looking knot, but make it neat.  After the knot is tight, clip both the extra fly line and the leader.No knot is made in the fly line it's self, as the knot in the leader will not slip off the fly line.  This works much better than those little eyes you push into the fly line.