One problem with the google app is that it will not focus. The auto focus keeps changing, so you just think it's sharp, then as you take the picture it blurs again. Open camera has a focus lock.
Its a good idea to have an uncluttered background. A white sheet of paper curved up the wall is popular. The auto-exposure will then give you a dark image. The model can be almost silhouetted at worst, a little muddy looking at best. The exposure needs compensating for the bright background. In Open Camera the brightness setting is right on the main screen. It will then use it's standard algorithm for guessing the exposure, and brighten it up from that baseline. Then you can see tonal variation on the model, not the tonal variation on the paper.
Here's a work-in-progress shot using Open Camera.