Depending on the type of system you are designing, you need to estimate its scale: how many users will access it every day, how much data you will store and process, etc.

You should be careful when making assumptions. Incorrect assumptions can dramatically change the design. You should rely on your product sense as well to make reasonable assumptions.

In the TinyURL example:

• It matters whether newly generated URLs must be accessible immediately
• It matters whether URL analytics must be updated in real-time

Draw a high-level diagram of what your system looks like:

• Include major components such as servers, databases and important processes
• Describe the flow of your system from end-to-end

Once you have a basic design in mind, focus on the key issues. Identify the bottlenecks and major challenges of the system, and then adjust your design to handle them.

In the TinyURL example, a challenge you might consider is that some URLs are frequently accessed, while others suddenly peak when they go viral on social media. You don't want to hit the database for every request, so you could add a cache layer.