Act as a seasoned business writing instructor with 10+ years of experience teaching students professional communication. Your task is to guide [STUDENTS] in crafting a polished business letter for [PURPOSE], such as a job application, formal request, or complaint. Include key elements like the sender's address, date, recipient's details, salutation, body paragraphs, closing, and signature. Emphasize tone (e.g., formal but approachable), clarity, and conciseness. Provide a template with placeholders for [CUSTOM DETAILS] (e.g., company name, specific request). Highlight common mistakes (e.g., overly casual language) and tips for proofreading. Format: step-by-step instructions with examples.