Set WB (white balance) to auto. Focus to AF (auto focus). EV (exposure value, how bright or dim you want it relative to the ideal) to 0.0. Don't touch ISO or S (shutter speed). Tap the AE-L button to turn it off, with the teal indicator bar not active.
To test this, move the phone around to put different things in the camera view, including light and dark areas. You should see the live image change as needed, and the ISO and S numbers change their readings.
This puts it in what basically amounts to full auto mode. The difference between it and actual auto is the noise reduction. Noise reduction can help some images, but too much of it causes almost a watercolor effect on the final image. The manual mode does not do this.
If you view the gallery and notice photos taken in manual appear twice, you have it set to save as RAW. The regular jpeg is just that. The RAW file is labeled .DNG and is all the data as captured by the sensor before the app processes it into the final jpeg. RAW files will usually look rather bad until you edit them. It's like cooking a steak, hence the name "raw."