Megabackdoor roth or backdoor roth?

Sorry if this question has been asked a few times here.

Our HHI is greater than limit to contribute to roth. I have never done megabckdoor roth or backdoor roth before and I am a bit nervous to do so on my own

1) How do I know which one I should be doing? Backdoor roth or megabackdoor?

2) Does this need to be done by Dec 31 or Aprik 15 ( tax deadline)?

3) Few year back when my income was less and I was not married I opened a roth In vanguard . Can I use the same Roth IRA account to contributeb$7k for mega backdoor /backdoor roth?

4) Can I just call Vanguard / fidelity and they can help me do this on the phone?

Please advise.