I'm going to use a piece of parachute cord for my teaching example. It's bigger than fishing line.
I'm also going to use a pair of scissors to show the hole in the hook.
Once string is through hole, it needs to spin on itself. Probably a good seven spins.
This spinning creates a small hole next to the scissors.
At this point, you have your spins, and the new hole. Take the free end of the string and insert it into the new loop.
This creates another loop, and the free end once again goes into the hole created .
Once end is through second hole and then the not free end is pulled
This will cinch the spins into a tight knot that should not untie.