If you donā??t even know how to prepare one you should have let it go. If you insisted on keeping it, killed it with a swift blow to the head. Instead it is slowly dying in a small pool of water because there is only so much oxygen in that water.
Donā??t just drain the sink to kill it, how would you like to be held under water. Kill it with a swift blow to the top of its head with a blunt, heavy object whilst holding the fish down. Something that looks like a police baton is the best.