Net the fish out and place it on a couple of paper towels folded over on a hard surface (table, bench, whatever). Give it a hard smack, or a couple. I use my hand, some prefer sticks. It is instant, no suffering, nothing. There is no other method as quick or humane, except cutting off the head but that is too much even for me. This method was rated "1" in an article on the topic in either Amazonass or Practical Fishkeeping a couple years back.
Those who advocate freezing, clove oil, and similar may find it less distasteful, but the poor fish is cognizant of what is going on. I don't treat any animal that way, not even a fly, it is whacked and instantly killed.