Best practices for writing user guides for a GUI include:

1. Know your audience: Understand who your users are and tailor the guide accordingly.

2. Clear and concise language: Use simple, straightforward language, avoiding technical jargon.

3. Step-by-step instructions: Break down complex tasks into manageable, numbered steps.

4. Visual aids: Incorporate screenshots, diagrams, and videos to illustrate key concepts.

5. Consistency: Use consistent terminology, formatting, and styling throughout.

6. Task-oriented: Focus on specific tasks and user goals, rather than features.

7. Concise headings: Use clear, descriptive headings to help users navigate.

8. Active voice: Write in the active voice, directing users to take action.

9. Feedback mechanisms: Provide opportunities for users to offer feedback and suggestions.

10. Updates and revisions: Regularly review and update the guide to reflect changes.

Additionally, consider:

- User testing and feedback

- Context-sensitive help

- Accessibility guidelines

- Translating guides for multiple languages

- Creating a glossary or terminology section

- Providing troubleshooting tips and FAQs

- Offering multimedia resources, like video tutorials

By following these best practices, you can create user guides that effectively support users in interacting with your GUI.