Fitting One Image Into the Other
We now need to insert one of the TV images into the display area of the
TV on the other layer. We have two identical layers, we will be manipulating
the upper layer to fit it into the TV on the lower layer.
We will be resizing and trying to fit one image into the other, this will be
very confusing for us visually. We will need to see the lines of the display
area of the lower TV through the image we will be manipulating - we can
achieve this by making the TV on the upper layer (the one we are manipulating)
transparent. But this approach will confuse us because the two images are
identical and too visually cluttered. (If this is not making sense, don't worry)
To solve this problem we need to temporarily fill the upper layer with
a solid colour, as well as make it transparent. Go to the Layers window
double click "Layer 1" until a "Layer Style" window pops up. This window
can be used to apply very interesting effects to layers, but we are interested
only in the "Color Overlay" effect. From the list of effects on the left, find
and check off "Color Overlay". Pick a colour to fill your upper layer with,
I chose red. Click OK.
The below picture shows you part of the "Layer Styles" window and the option
you need to choose.

Now that the upper layer is filled with a colour, we need to make it transparent
so that we will see the image underneath. Remember that these effects
are temporary, and we will undo them later. Set opacity on layer 1 (top layer)
to about 60%, the opacity can be set using a slide at the top of the "Layers"
window. Below is a picture of that window, showing the effect being applied
to Layer 1, the opacity being changed and, at the left, the bottom image
showing through our red layer.
Now, we need to transform the red layer and fit it exactly into the display
area of the TV that is underneath.
Go to EDIT->FREE TRANSFORM while Layer 1 is selected. ALT-CLICK on one
of the corners of the red square, drag it to the closest corner of the TV's display
area. This is what it should look like:
Keep ALT-CLICKing and dragging the other corners of the red layer over
the corners of the TV screen.
You are done when all four corners of the red layer are lying on top of the
four corners of the TV screen. Well done.
Remember that every single action that you have made has been recorded
by Photoshop so far. You will not have to do this process over for every
consecutive TV that you would like to insert into the screen.
Because filling the upper layer with red, and making it transparent was only
temporary, we need to reverse these effects. Go to the Layers window,
under Layer 1 there will be a tab with effects that have been applied to it,
untick the "Effects" checkbox (next to the eye icon) and make the opacity
100%. Below is what your Layers window should look like. You now have
one TV within the other.
We're almost done. To make repeating this process easy, we need to combine
the two layers into one, merge them. Go to LAYER->MERGE VISIBLE.