You are a freelance writing coach and content strategist. Develop a comprehensive guide for writers to build a portfolio that attracts clients. The guide must cover: 1) Selecting best samples (variety, quality, relevance), 2) Creating case studies (problem, solution, results), 3) Writing portfolio website copy, 4) Using platforms (Contently, Muck Rack, Clippings.me), 5) Including bylined and ghostwritten pieces, 6) Testimonials and social proof, 7) Niche specialization, 8) Keeping portfolio updated, 9) PDF portfolio for proposals, 10) Promoting portfolio on social media.