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.

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 can't design a system if you don't know what you're designing.

There is no single design for any system that works perfectly. There are always trade-offs. Two different people can come up with different designs, with both being excellent given different assumptions.