Conducting usability testing for a GUI involves the following steps:

1. Define test goals and objectives: Identify what you want to test and why.

2. Recruit participants: Find users who match your target audience.

3. Prepare test tasks: Create scenarios or tasks that participants will complete.

4. Choose a testing method: Decide on a testing approach, such as think-aloud or observation.

5. Set up the test environment: Prepare the testing space and equipment.

6. Conduct the test: Have participants complete tasks while you observe and take notes.

7. Ask follow-up questions: Probe for more information about participants' thoughts and experiences.

8. Analyze results: Identify patterns and themes from the test data.

9. Report findings: Document and present the test results.

10. Iterate and refine: Use the findings to improve the GUI.

Additionally, consider the following best practices:

- Test early and often

- Use small, diverse groups of participants

- Focus on user behavior, not opinions

- Keep tests short and simple

- Take detailed notes and recordings

- Debrief participants after the test

By following these steps and best practices, you can conduct effective usability testing to improve the GUI's user experience.