I used to have two gouramis that would eat out of my fingers and nibbled on them. it didnt really require any "training". first time I tried it they just came up and started nibbling.
I have 2 goldfish that nibble my fingers if I stick my hand in their tank. If I dont have bloodworms for my ADF, (I hand feed them) they swim away. If I do have bloodworms, they let me pet them with my other hand while they eat out of my fingers.