Taking a good shot
The better the photo the better the playback. The most important things are strong lighting, good focus and a straight image taken from a flat page.
Tip: it is often easier to photograph the music upright. If you put the music flat on a table there is a risk your device camera will rotate and you will capture the music on its side. Something like music stand is ideal because that helps keep the music flat.
You should get results from any well lit, well-focused picture of a page of music. But it is worth a little experimenting with lighting and positioning. Generally speaking the more light the better. But different devices have different characteristics.
Focus is equally important, and the steadier you hold your device the better. Some devices take a few seconds to reach best focus, so hold the device steady while it adjusts. You may find it helps to rest your arm on an object such as the back of a chair. You will find that you improve with practice. When you start, try different shots of the same page and get a feel for what works well.
The left edge of music carries a lot of information and for music with more than two or three staffs it helps to photograph these clear and straight.
There is no need to fill absolutely the whole frame with music. Avoid getting so close up that the picture is distorted. Use the screen shots as a guide. Sometimes you can improve results by backing off a little do that the music fills less of the page.
If you find there is something wrong at a first attempt, persevere with more shots. Results will improve.