Open Photoshop. Click the "File" menu and select "Open". Browse to the photo to use for the base, upon which the other photo will be superimposed. Double-click the image's file name.

Click the "File" menu again and repeat the "Open" process. The second picture opens in a new tab on the Photoshop work area.

Pull down the "Select" menu at the top of the screen and select "All". Press the "Ctrl" and "C" keys on the keyboard to copy the picture.

Click the original picture. Press the "Ctrl" and "V" keys to paste in the image. Depending on the two image's orientation and sizes, this second picture may now be completely obscuring the first.

Right-click the "Eraser" tool, which looks like its name, in the middle of the "Tools" palette. Select "Background Eraser Tool", which looks just like the "Eraser" but has a small scissors on it, from the fly-out menu.

Position the cursor on the photo. Press and hold down the left mouse button. Slowly drag the cursor over the picture. The photo below will start to show through from the bottom as you "erase" the image on top. Continue erasing until the two pictures are superimposed together as desired.