You just tap the microphone in the bar above the keyboard to enable it.
I have a question also, about this feature. I read in another post, can't remember which, that you could "speak" the punctuation like "comma". However, when I do, it only inserts the word "comma". However, when I dictated, "One comma two comma three", it translated it to "1 comma 2,3", so apparently it works part of the time.
It would be nice if I can say, "colon", "comma", "period", "point", "exclamation point", etc.
Although, "hashtag" followed by something makes a twitter hashtag, which is nice.