First, using either the Quick Selection tool or the Lasso tool, select the piece of the first image you would like to use.
Then drag the selected piece of the image onto the second image you would like to lay it over.
Using Edit > Free Transform, size the image however you like. If you hold down the Shift key while sizing the image, it will keep its original proportions.
Click the “Add Vector Mask” icon, located at the bottom right of the screen and appears as a rectangle with a circle in it. Then click the “Gradient” tool along the left side of the screen. Then click the “click to edit gradient” feature at the top left of the screen. It appears as a color bar with a drop down option. Click OK to the screen that appears.
In order to impliment the gradient, you must click and hold the mouse where ever you would like the gradient to begin and drag it and release click to where ever you would like the gradient to end. There are several options for what you can do with the gradient in terms of its shape. For this specific image, I used the “Radial Gradient” option in order to have a circular gradient.