How can I learn to be a good product designer?

Before you start to learn product design, there are 2 things you can practice to prepare yourself.

Tell the nuance
It's commonly said that a taste is essential for a designer. Taste is obsession about telling the difference that others couldn't see. For me, this is a skill that can be acquired rather than a gift.

Telling the visual difference might be the most straightforward thing you can do. Rarely can you find two nearly identical objects and compare them with each other, like what you do in those Spot the Difference puzzles. But you can always start to look closely at things around you: their compositions, colors, alignment, spacing, typefaces, or even typos. Compare them with others and feel the difference. When you can tell the difference but can't know why, it's time to learn some basic visual principles.

The same technique also applies to products. Pay attention to every product you see everyday -- it's much easier to learn about product design when you can see the difference in detail. After you see it, think about why: Why does this device have the button here? Why does that app have that feature there? It's important to back-step the difference you see, so that you can make your own product decision better in the future.

Have an idea
It's crucial to always have ideas for products, not necessarily a "design idea" but rather an obsession to make your own life better.

Take a closer look at your daily routines and ask questions: How can I avoid just missing a bus? Should I wait for the express train when the local train comes more frequently? Why do I need to always unlock my phone, check the app to know the weather when I have a window to outside right in front of me? What can make them better?

These inconveniences in your everyday life, though small, could be turned into design challenges. This is where you can start thinking about improvements: Can I make a device that measures my walking speed and tells me if I'm walking fast enough to catch the next bus? Can I make a data visualization of local and express train running on a map according to the time table? Can I attach a tiny screen to the window glass so that I can see the forecast where I naturally look out for weather?

Then you'll have the product ideas that you can learn to design for.