Sorry you have to do that. It's always hard.
Well, the way i do it (if i have to).
I put the fish in a small container of water, just big enough for it to turn around in.
Then i add 2 Alkaseltzer tablets for each 5-10 gallons of water. This overloads the water with carbon dioxide.
This will cause rapid unconciousness and then death.
Fish should be removed only after ten minutes have passed since their last observed opercular movements (respiration) have occurred.
This works rather well and is very humane. THe fish basically goes to sleep. There is no pain involved.