How to Tie a Fly Line to a Leader
To tie a fly line to a leader, start by making loops with both lines and pinching the fly line in your non-dominant hand. Then, insert the fly line loop through the loop in the leader, and grip the leader loop in your non-dominant hand. Next, make a second loop in the leader with your dominant hand, and thread it through the welded loop in the fly line. Finish by pulling the loose end of the leader free before pulling on the leader and fly line to tighten the knot.
Buy a leader with a perfection loop. Visit your local fishing store, and ask a store representative to help you select a perfection-loop leader. These ready-made loops are created by the manufacturer and often more secure, not to mention less of a headache, than making a knot in your own leader. Buy a leader between 7.5–9 feet (2.3–2.7 m) (2.29-2.74 m) long. Browse different leader packages to find one of this ideal length. Lengths are often highlighted on the front of the box in the top corner. A leader longer than this may be difficult for a beginning or intermediate angler to manage. A leader shorter than this may make your presence too obvious to the fish. Buy a fly line with a welded loop. Ask a fishing store representative to recommend a fly line with a welded loop. These loops are welded shut by the manufacturer rather than tied. This gives your fly line additional strength and durability. Ask a store rep to help you select a fly line of the right length and weight. Describe your typical angling session, rod heft, and fly-fishing ability to a store professional. Based on this information, they can help you select a line of the most appropriate length and weight for you. Your ideal fly line length and weight depend upon the water depth you plan to fish, the weight tolerance of your rod, and your angling skill. It can help to bring your fly fishing rod to the store with you if you're unsure of its weight-bearing abilities. The representative can determine the capabilities of your model. Open the leader package. Gently pull the coiled leader from its package, retaining its circular shape. Resist the urge to uncoil the leader as you unpack it, as this can create undesirable knots. Place the coiled leader around the 3 middle fingers of your non-dominant hand. Hold the index, ring, and middle fingers of your non-dominant hand straight with a little space between each of them. Touch your pinky and thumb together across the palm of your hand, so they are out of your way. Slip the coiled leader about a third of the way over your 3 fingers, and apply gentle pressure to the interior of the circle. Your fingers will act as a spool or anchor to help unwind the leader smoothly. Uncoil your leader. Grip the perfection loop of the leader with your dominant hand. Gently unravel the leader, using your 3 fingers to hold the coil of leader steady. Your leader will unspool into a long length of thread. The perfection loop will be slightly loose from the rest of the coil upon unpacking. You'll recognize it because it is the thickest part of the leader. You may need to work through a few places where the leader wraps over itself in the coil. Work through these patiently as not to cause knots. Relax your hands as normal with the unspoiled leader in front of you. Pull the kinks out of your leader. Holding one end of the leader firmly between your fingers—it doesn't matter which end—use a pinched grip with the index finger and thumb of your other hand to run over the length of the leader. Apply pressure as you do this, which will create some warmth from the friction. This warmth will help smooth out any remaining kinks or coils in your leader. Running your fingers over the leader in this way also removes shine and makes the leader less obvious to the fish. You can use a leader straighter rather than your fingers to remove kinks, if desired. Leader straighteners can be purchased at your local fishing store. Pinch the fly line in your non-dominant hand. With your dominant hand, pinch the leader. Grip each item between your thumb and index finger about 1 inch (2.5 cm) below their respective loops. Insert the loop in the fly line through the loop in the leader. Keeping your pinched grip on both sides, gently thread the loop in the fly line through the loop in the leader. This should be relatively easy, as the fly line loop is typically smaller than the leader loop. Slide the leader down the fly line until the perfection loop hits your pinch. Open the fingers of your non-dominant hand slightly, and add the leader loop to the pinched grip of that hand. You will now be holding the leader and fly line in a pinched grip in the fingers of your non-dominant hand. Release your grip on the leader with your dominant hand. Create a new small loop in your leader. While still pinching the leader and fly line in your non-dominant hand, grip the free length of your leader about 2 inches (5.1 cm) away with your dominant hand. Create a small loop, and pinch it between the index finger and thumb of your dominant hand. The new small loop should be about the same size as leader's original perfection loop. Thread your new loop through the welded loop in the fly line. Press the top edge of the new loop you've created in the leader through the welded loop in the fly line. Keep pushing the new loop through the welded loop until you can pinch the new loop of the leader on the other side. Continue to secure the leader and fly line with the pinch in your non-dominant head. Pull the end of your leader free. With the fingers of your dominant hand, grip the top of the leader that's been threaded through the welded loop. Pull the loop, increasing its size, until you pull the loose end all the way through. Loosen the pinch of your non-dominant hand. Soften your grip on the leader and line in your non-dominant hand, and let go of the leader. Maintain your grip on the fly line with the non-dominant hand. With your dominant hand, regrip the leader about 2 inches (5.1 cm) from your non-dominant hand's pinch on the fly line. Pull both ends tight. Apply gentle pressure, pulling both sides firmly away from one another. The perfection loop of the leader will interlock over the welded loop of the fly line to resemble a square knot. Give the two sides one final tug to ensure your new knot is secured snugly. You're ready for your next fishing adventure! 
Buy a leader with a perfection loop. Buy a leader between 7.5–9 feet (2.3–2.7 m) (2.29-2.74 m) long. Buy a fly line with a welded loop. Ask a store rep to help you select a fly line of the right length and weight. Open the leader package. Place the coiled leader around the 3 middle fingers of your non-dominant hand. Uncoil your leader. Pull the kinks out of your leader. Pinch the fly line in your non-dominant hand. Insert the loop in the fly line through the loop in the leader. Slide the leader down the fly line until the perfection loop hits your pinch. Create a new small loop in your leader. Thread your new loop through the welded loop in the fly line. Pull the end of your leader free. Loosen the pinch of your non-dominant hand. Pull both ends tight. 