What is the proper way to address a letter to a business?

• Your address at top left corner
• Date
• 3 spaces and then the address of the company
• If it is to a specific person and you don't know their name or if they are male or female use: Dear Sir/Madam
• If you do know it's a man then you can put ATTENTION: Mr. Robert Smith
• If you do know it's a woman then put ATTENTION: Ms. Sue Smith

If it's to the whole company put: Management & Staff (then address under that.)