1. Open the photo.

2. Double click on the background to make it into a layer.

3. Add a gradient mask to cover part of the photo. Change to the gradient tool in the tool box. Set foreground color to the color you want for the opaque part, and in the options bar, choose the Foreground to transparent gradient preset (you may need to load the default gradients to see it.)

4. In the options bar you may need to reverse the gradient. It depends on your photo. I dragged from the top of my photo to the bottom with the gradient tool twice. Adjust where you drag to make the mask appear where you want it.