How do create a mockup ---> insert an image into another image

If you want to follow the  instructions in the video, before you begin choose View > Rulers. From the rulers now at the top and side of the image drag out the reference lines that were are referred to.

An alternative to the method described, consider the following,

1. Choose the Podlygonal Lasso tool (it is nested with the Lasso tool). Click and drag touching the four corners of the white area that will hold the new image. You will see a marching ants tilted rectangle.

2. Open the new image.  Its proportions should be similar to the white panel area proportions. If not crop as needed. Then Select > All.  Edit > Copy.

3. Return to the original image. Choose Edit > Paste Special > Paste Into. The result is the bottom left image

4. Choose Edit > Transform > Rotate so that the tilted image content aligns with the iPad black frame. Don't OK yet. (Fig 3)

5. Choose Edit > Transform > Distort and use the four corner handles to bring the four corners of the new image into the white frame area altering its size.  (Fig 4). Note the jntentional image distortion created by matching the tilt and twist of the iPad.

ALSO: If, because of the size of the new image, the Transform handles are outside the screen viewing area, use Cmd+0 (zero) to change the view so that the handles can be seen.)