The core principles of "How to Win Friends and Influence People" by Dale Carnegie focus on building strong relationships by genuinely appreciating others, avoiding criticism, and showing sincere interest in people. Key strategies include smiling, remembering and using people's names, being a good listener, talking about others' interests, and making others feel important sincerely. To influence others, one should avoid arguments, respect opinions, admit mistakes quickly, start friendly, encourage others to talk, and appeal to noble motives. Leading and changing people without offense involves praising improvements, calling attention to mistakes indirectly, encouraging and making it easy to correct faults, and giving people a fine reputation to live up to.