For the best results when taking a photo of a photo with your smartphone, you need to get it lit adequately. Too much lighting and your pictures will he bright spots of light, too little and they will look underexposed.
The best lighting you can get is natural lighting. So, the best thing to do is place your picture somewhere near a window, skylight, door, or any other source of natural light indoors.
Otherwise, you can just head outside and take the photos there. If you decide on that option, it’s best to take photos when it’s overcast because the lighting is less harsh.
If you’re taking photos indoors, you can also use the house lights to add some lighting on the picture. Just don’t place the photo directly under a light or point a light straight at the picture you want to take a photo of.
A direct light from above when taking a flat lay photo will result in shadows being cast on the picture. Also, shining a light directly at the photo will create a hotspot or a bright area where the light is reflected, especially if the picture has a glossy finish. Instead, shine the light off to the side and he the picture illuminated with soft light.
Most importantly, do not use the smartphone’s flash when taking a picture of a photo with your phone. It will reflect directly on the surface of the picture you’re trying to capture and cause a bright spot. Not a good look if you want to make the photos look as best as possible.