With keyboard dictation, you can dictate text anywhere you can type it. You can enable this by going to System Preferences > Keyboard > Dictation. Select On and then choose which shortcut to use to begin dictating using the Shortcut menu. By default, you press the Fn (Function) key twice to start or stop keyboard dictation. You can also customize your own unique shortcut from the same menu.
To use Dictation simply place your cursor where you would like to begin dictating and then click your chosen dictation keyboard shortcut or select Start Dictation from the Edit menu. A small microphone icon will appear with the word Done underneath it. Simply begin speaking. When you're done click Done under the microphone icon or press the escape key.