Apple website about how to use Dictation?

Sure.

1. Use Dictation Commands to tell. your Mac what to do.
2. Use your voice to enter text on your Mac.
3. Commands for dictating text on Mac.

If you use the default dictation, it will send your spoken words to the Apple Dictation server. Although more accurate, this can be effected by network conditions and may be sluggish at times.

I use Enhanced Dictation, which performs voice interpretation on the Mac, and not over the network. This will be less accurate, but more responsive.

When you first launch Dictation (I use fn fn), you must wait for the pulsing dots to finish and the microphone to appear before speaking. The first thing you should say is "Show commands." — without speaking the quotes. This will give you a good list of available commands. You must click on the application that you wish to dictate into before you start speaking or you may hear a beep, and your words will not appear.

Dictation will over time, learn your speech patterns, provided you do not sit too far away (leaning back in your chair) from the microphone, and speak clearly. It will frustrate you to no end though.