A lot of things can be learnt using books and mentors but as far as selling is concerned you might as well be asking "how did you learn to swim?"

It's the same answer.. you learn to swim by swimming. You can ask all the people jumping into the water on which technique is best and what hacks are the best.. but even after all the advice you will get shocked and struggle with it when you jump in the first time and it will take a lot practice before you do the first lap.

So depending on the type of profession there is no better way to pick up the phone, write that email or start connecting with the right people (irl or online) and just get started to learn it.