It' easy. You were just confused with the function of the comma.
「でも、から始める話し方は直した方が良いよ」
=「
『でも』から始める話し方は直した方が良いよ」
=「"
でも"から始める話し方は直した方が良いよ」
=「"
でも"で始める話し方は直した方が良いよ」
=「
否定的な意味を持つ逆説の接続詞の"でも"から始める話し方は直した方が良いよ」
=You had better change your talking style, which (always/often)
starts with "But."
=「否定語(逆説の接続詞)で話を始めない方がいいですよ。」
The context is something like this:
You always/often say "But ......."
It sounds very negative and arrogant. You look like denying everything, which is very bad.
You should change the bad habit.
I advise you to start your sentence with an affirmative comment.
For example, "
That's true, but...."