Hay Jay,
you could certainly buy a fancy heat gun and I have used an old heat gun of mine that I used to strip paint works well. But… I like to use an ordinary tea candle. I find I can control it easier. You do need one other thing for this and that’s some narrow light clear shrink wrap tubing a bit wider than your fly line.
This method is great for all coated fly lines and gives strong, small and neat loops, which will glide noiseless through the guides. Here’s a step by step, if you have an old line you can use it to experiment with. It does take a bit of patience and practice but not hard to do.
1. Cut a piece of tubing approximately about 2 to 3 inches long. Pass the fly line through the shrink tubing so a few inches protrude out the other end.
2. Loop your fly line and pass it back through the tubing you should end up with 1/2 inch from the end of the shrink tubing. Adjust the loop size to your liking
3. Slowly apply heat to the shrink tube with the side of a candle. Heat the line in short intervals and gently. The coating of the fly line will melt together. Only heat the shrink wrap tubing not the fly line protruding. Repeat this slowly until all the fly line connection is smoothly merged and melted together with no gaps between the two lines, take your time and work at it. I like to roll the melted line between my fingers a bit when it is still warm. careful not to burn yourself.
4. When the coating of the fly line cools a bit, you can remove the shrink tube carefully. The shrink tube is best removed while it is still a little warm. I use a fine pair of scissors and cut from the loop down.
5. Don't apply too much heat or you will burn the line. If the line burns or browns clip it off and start again.
Hope this helps and makes sense. Started using this method a years back. Never had one fail, just like factory made loops.