When you buy a tuner, there is something like a clip. The wire is inserted into a small hole on the tuner. Install the battery in the tuner and clamp it on the piano board.

There are three light bulbs on the tuner. Play a note. If the left side lights up (red), move the code to the right. If the right side lights up (red), the same principle applies. If the middle side lights up (yellow), it means The tone is accurate, you can also tune it with a wrench

When tuning, pay attention to whether the string tone displayed on the tuner is correct (that is, 25 strings, counting from bottom to top)