Primary sources offer you the chance to engage with a true piece of History - instead of taking a textbook's word, you can formulate your own opinion on the matter. Start by asking yourself what the source is saying, on a basic level. Identifying the immediate message will help you to understand the reception it may have received. For now, forget about your own knowledge on the era, and focus on what you can learn from just the source. This will be your first paragraph.
Now it's time to bring in your own historical knowledge. Think about how the source compares to any other primary sources you have seen, and think about whether it corroborates or contradicts your own knowledge. This will help us to discuss the "why?" section. Consider the possible motivations the source might have. This will help us to decide its usefulness. This can be a second paragraph.
Finally, think about how these observations give us an insight into the period. Consider how the source has expanded your understanding, and note where it has been less useful. This will form a conclusion.