method 1. betta box with mirror and food inside. feed daily from inside the betta box. wait till fish go in, get comfortable, close it and ta-da
method 2, stun method. locate ur fish when it sleeps at night. when tank lights are pitch dark, blast ur main lights on. the sudden light will stun your fish for a few minutes, rendering them extremely easy to catch. simply scoop them up. this method works 95% o the time if you are quick enuf to get the fish out before it regains consioucness and swim away. this method does not work for fishes sleeping in deep caves and crevices. a stick might be useful to prod the stunned fish out.
HTH. works all the time.