How do i make my images not look cut out?

My go to first procedure for changing the lighting angle.

Original

Select the Quick Select tool and click on Select Subject in the Options Bar

Copy the resulting selection to a new layer, and set to Screen Mode

Note This is exactly what Select Subject selected.  There is zero fine tuning.  An amazing job, and kudos to the Development Team.

Add a layer mask, and use the Gradient tool set to Foreground to Transparent

This lets us 'build' our gradient using multiple drags across the image.

Two hits with the gradient tool, and we just have the left flank left, but we still need to kill the highlight on the front 'car left' corner.

What I did here is Ctrl click the copied car layer to select it as a selection, and with that selection place, I opened a curves layer above the background, but below the copied car layer.  The layer mask automatically restricts the curve to the car.

A bit of fine tuning, like mask out the back of the mirror and fine tune the roof line, and we have reversed the lighting direction

As Dave says, perspective is a big factor.  In our Something For The Weekend challenges — which ChelseaW and anyone else are welcome to join and contribute to — a good part of the time it takes me to complete an entry is finding suitable source images with the right lighting and perspective.

Shadows are probably a bigger factor thinking about it.  Let's try an example, and I am knocking this together as I go just for this thread.

Dropping the car (which I have roughly cut from its original background with Select Subject) into this road scene, we can see they are massively mismatched, but let's see what we can do.

The first thing we can do is flip it horizontally to match the scene, but the only place it will fit is part way round the bend.

So we need to select the foreground that would conceal part of the car, and copy to a new layer, and place this layer above the car.

I pretty much _always_ work this way, as it means I can experiment with the car, and move it about, without having to remake any selections.

Another point here is that I always try to overlap part of the original above the composited element. This gives it depth and makes it belong.

The car is now on the road, but much too big.  So we make it a Smart Object (by right clicking the layer). That way we can keep changing the size without loosing quality

The car is now about the right size, but obviously does not belong there, so we need to add some shadows to tie it to the road.

The easiest, and usually best, way to do this is

1. add a layer below the car layer
2. Ctrl click on the car layer to load it as a selection
3. Fill the selection with black in the new layer.
4. Move this new shadow into position and blur with Gaussian blur

This gives us a problem in that the shadow should not project from under the wheel, so we add a layer mask and paint that out.

Now we have to fix the lighting on the car, which has far too much contrast, so we need to kill those highlights with a curves layer.

We are actually pretty close, and only took a few steps, but there is one glaring problem that I will _attempt_ to fix

The near side front wheel is pointing straight ahead

I need to try and making it follow the curve (to some extent at least)

This is where using a Smart Object for the car comes in handy, because we still have the original, larger size, file to work with.

Double click the SO, and it will open in a new layer

The important part of this trick is squash the wheel to change its perspective

It is then just a mater of painting in (and out) to make the rest of the wheel fit this new perspective.

I actually used the squashed wheel twice, to help fill in the new tire wall position, but still needed to hand paint in some making good.  Freehand brush work is an essential skill to learn

Finally I Ctrl clicked the new wheel shape to load as a selection, and stroked it with white for a highlight.

This was blured, and masked out at the bottom of the wheel

It would be better with more turn on the wheel, but this works as a quick demo.

I hope this has helped a bit.

I'll leave you with a trick to soften outlines:

1. Ctrl click the object to load as a selection
2. at the bottom of the paths panel, click on the select to path icon

Set up the Blur tool with the right sized brush and strength (I have set to 100%)

Right click the Path, and use the drop down and select the Blur tool

Having set the blur tool as the default stroke, you can repeat by clicking on the Stroke icon at the bottom of the paths panel.