Unless it's a trophy fish, unhook the fish in the landing net under water, subside the net, and wait untill the fish swims away.
If it's a trophy, don't unhook the fish, keep in the landing net untill someone has the camera ready, lift the fish out of the net, take the pic immediately and put the fish back in the net, underwater. Unhook the fish, then place your hand under the fish, which is still in the net.
Turn the net upside down, still cradling the fish, and "swim" it backwards and forwards untill it kicks out of your hand and away to freedom.