1. Define the central theme: take "Lanzhou" as the central theme and draw a circle to represent it.

2. Add sub-themes: around the central theme, add four sub-themes, namely "history and culture", "food", "tourist attractions" and "special food".

3. Add secondary branches: around each sub-topic, add secondary branches related to it. For example, around "gourmet", secondary branches such as Lamian Noodles, beef noodles and mutton kebabs can be added.

4. Explore more branches: According to the topic you want to explore, add relevant branches continuously. For example, around "history and culture", branches such as Lanzhou University, Lanzhou Grottoes and Lanzhou Museum can be added.

5. Optimize the map: With the increase of branches, you may need to make adjustments to make the map more clear and readable. For example, you can use color tags to indicate different types of topics and branches, or you can use icons or short words to emphasize key points.