To improve the performance of your managers, start coaching them. Whatever you tell them, they are unlikely to learn it well enough. Skills won't be developed unless you start reinforcing theoretical knowledge with practical exercises. Set aside at least half an hour a day for this. Training can be very simple. For example, one day managers practise the process of finding out what a customer needs. One manager plays the role of the customer, while the other has to find out what the customer needs.
Incidentally, the stage of identifying the customer's needs is often undeservedly forgotten by sales managers. Or they deliberately skip it because they don't understand how to go about it. Do you know why managers do this? It's fear of asking questions. It takes many hours for a person to learn how to ask questions easily, naturally, without effort. You also have to learn to listen to the answers and analyse them quickly in order to give the client the information they need or to ask clarifying questions.