It's not that simple. There's building blocks to getting a horse on the bit.
You also need to understand what it means.
Here's an article that will explain it much better than I can. As you can see, there's quite a bit of work a horse and rider have to get down before asking for more (being on the bit counts as one of those "more" things).
The article breaks it down into three aids but your horse has to be able to understand all those aids correctly before they're applied like this.
I'm going to guess you're not working with a trainer. (Nothing wrong with that. I'm without one too and so are many of us) If you're trying to ride and train dressage then its important that your Sally read and educate yourself on terminology and theory. 3 Aids To Teach Your Horse To Go On the Bit & Round | Regarding Horses Posted via Mobile Device