How can I speak Mandarin well?

That depends on your situation. I don't know.

First, you can't tell the pronunciation at all, which means you don't know the correct pronunciation of every word. If that's the case, then you can look it up in the dictionary and read and practice with the textbook with phonetic notation used by primary school students.

Second, the pronunciation is clear, you can read, but you can't, or you can't read fast. This is easier to handle. Read newspapers and magazines every day. Practice makes perfect.

Third, you can pronounce clearly, but some of them are illiterate and inaccurate. For example, flat tongue and upturned tongue, side nasal sound, front nasal sound and back nasal sound are all typical. This situation is really difficult to correct. You should listen to what others say on TV, consult your classmates if necessary, and strengthen pronunciation training before you get familiar with it.

It's no use just listening. You listen every day. Speak more and practice more every day. You can put a candy in your mouth when you practice. Practice can also be gradual. You can read the words first, then the words, and finally the article every day. Speak more Mandarin at ordinary times, even if it is not good. Cultivate a sense of language. If you can persevere, I absolutely believe that you can speak fluent Mandarin within one year.