If your freezer is very cold, putting them in a small container of water and freezing them will work quickly and (for most fish) cause little harm, but it is considered malpractice in lots of places for a vet to reccomend it because studies have found electrical activity in the unconcious fish after the water has frozen.
I tend to put them in a tall narrow container with as much water as it takes for them to be able to swim and drip vodka in at a fast rate, aiming for a 20-25% solution of 35%-45% ABV Vodka (when I have stronger alcohol around, like the stuff for cleaning lab equipment I use that instead). The aim is to get the alcohol to 7%-12% ABV in the fishes water quickly, without the fish contacting any sections of over 20% alcohol, I know some people who make up a weak solution of vodka and put the fish in it, but I don't like the thought of my fishes last moments being in a net and then dumped into a bizarre environment, I prefer to let them calm down in the container and then bring on the euthanasia.
Once the fish appears dead, I follow up with freezing, because it has been known for brain damaged fish to recover if kept moist.
Another recommended method is decapitation, but I don't have the heart or stomach for that.