Follow the steps below:

1. Log in to Doudian backend: Use your Doudian seller account to log in to the Doudian backend management system.

2. Enter product management: In the backend management system, find the product management option and click to enter the product management page.

3. Edit products: On the product management page, find the product you want to add instructions for use and click the Edit or Modify button.

4. Find the product description area: On the product editing page, find the description area, which is usually a text box or rich text editor. This is where you can add product instructions for use.

5. Add instructions for use: In the description area, type or paste your instructions for use. You can use text formatting options, such as bold, italics, numbered lists, etc. to make the instructions for use clearer and more concise.

6. Formatting and layout: Use the tools provided by the editor to properly format and layout the instructions for use to ensure that the content is easy to read and understand. You can use headings, paragraphs, sections, and icons to organize and emphasize different parts of the instructions.

7. Preview and Save: After you finish editing the instructions, click the Preview button (if available) to see the final effect. If you are satisfied, click the Save or Submit button to apply the instructions to the product details page