Some people say that there overlays look like stickers and they cant seem to get them to look realistic.
So here is a few tips and tricks to get those overlays to blend right!
FIRSTLY
When you place an overlay onto your image you should always take note of the lighting and what direction it is coming from.
In this image below you can see that this is NOT right.
So I will flip the car.

Straight away that looks much better!
Sizing is also an option you want to look at you dont want something looking too big or too small to the objects around it as this will make it look unrealistic too!

Now Select your overlay... Quick way to do this is hold Ctrl and click on the overlay it will select it with a line of marching ants.

Select you marquee tool and right click inside your marching ants. Select Feather for this image I selected 20px and hit enter. This softens the edges but some overlays are different.

Then I duplicate the overlay.
Then I select the bottom image and Ctrl T for transform.
I hold the Ctrl button while I move the image so I can distort it's shape. Taking note on the sun light in this image I position the "2nd" overlay to where I think it looks just right.

I then add an exposure layer above this and clip it.
I bring the exposure all the way down so it is completely black.

I then make sure I am on the "2nd" Overlay and I put a gaussian blur filter over the top. Depending on the surroundings of the overlay on how much blur you would use. I used about 30% on this image.

I then lower the opacity to what I think suits it surrounds and that's it!
I am done for this image anyways.

Some overlays require a lot more work as this is a pretty simple image.
But these basics are a great way to start.
If you have any Questions about adding overlays or need some help please leave a comment below and I will answer you a.s.a.p.
Thanks and have a great day :)