You can get high quality fly line at your local fishing supply shop or online.
Use thin backing line made for fly fishing, as it will be strong enough to work properly.
Maintain a firm grip on the backing line and the loop on the fly line with your thumb and forefinger. Make sure neither line slips or moves.
The backing line should cover the loop in the fly line as you wrap it.
Use your middle finger to guide the tag end over the loop so the backing line sits snugly against the loop. Wrapping the tag end over the fly line several times will ensure the knot is tight and strong.
Stop pushing the knot once it reaches the end of the loop on the fly line, as you do not want it to slide off the fly line.
You can also pull tight on the tag end of the backing line while pulling the fly line to make sure the knot is extra tight. To ensure the knot is secure, hold the tag end of the backing line with a pair of forceps or pliers and pull it tight while holding the fly line.
Be careful not to cut off any of the live end of the backing line.
Make sure you do not cut off the live end of the fly line, as this will ruin all your hard work.