To learn how to use manual mode you need to start by putting your camera in manual mode. Take photos with the camera's suggested aperture, shutter speed, and ISO combination. Then experiment with different apertures, shutter speeds and ISOs, and combinations of the three. And check out what has happened when you changed these things.

Take photos that are correctly exposed, underexposed, and overexposed. I did this and it helped me to understand what was going on. It helped me to understand the aperture, shutter speed, ISO, and how they work together.

When I applied what I had learned to my photos, the photos that I took started to improve. And that was the beginning of my journey to where I am now.