Have you ever done it? Most people say that egg yolk won't work, because the fry doesn't readily consume it and that it spoils the water quickly.
I usually use banana worms. All you need is water, oats, container and a spoon of starter oats. They are microworms that move in water and attract the fry to hunt it. My babies are one week old and growing quickly I'll post some pics soon too.
In the meantime if you like betta, I'm sure you can use this Tool