How can I pronounce the flap T sound in American accent?
How can I pronounce the flap T sound in American accent?

Is it /d/ or /r/? for example I think better sounds more like berrer (not bedder), also beautiful sounds like beauriful.

Is it standard and formal to pronounce this sound?

Question

How can I pronounce the flap T sound in American accent? 
How can I pronounce the flap T sound in American accent?

Is it /d/ or /r/? for example I think better sounds more like berrer (not bedder), also beautiful sounds like beauriful.

Is it standard and formal to pronounce this sound?

Answer

You're thinking too much about it. Just listen and imitate.

By the way, what you hear as an R is not heard that way by speakers of English.  We'd hear a D or a T.

Answer

Good question! :)

American speakers tend to use the /d/ in your examples (better = bedder). Definitely not "berrer".

British speakers tend to use the /t/ (better=better)