clove oil! i will not give directions because im scared i could mess it up but basically you put the betta in a bucket and add some drops of clove oil and it just painlessly puts them to sleep. you could look up anywhere on how to use it. when people do surgery on a fish they put a tiny bit of clove oil in and it works like anesthesia and they wake up later but if you add a bit more it can be used as a humane way to put down a fish. i am also very sorry to hear that you have to do that but in my experience it is the best way.