I think the main battle is mindset. The willingness to be seen attempting to persuade others to buy your product or invest in you or whatever. A lot of people, including engineer types, have a deep aversion to this, which may remain unexamined even after they’ve consciously recognised they need to ‘learn marketing and sales’.
I suspect success in this area might be only 20% about learning practical methodology. The other 80% would be some kind of self-examination, then analysing and resolving whatever aversions you discover - or deciding on a different strategy such as teaming up with someone who is already comfortable with marketing/selling.