The overlay sketches? Do you mean this sort of thing? If so, it really isn't anything clever or that technical.
I used MS Paint as it was free and easy to use and then did the following.
1) Find a side on picture of car 1 on the internet and take a copy or screen shot.
2) Open Paint and paste in the picture
3) Find a side on picture of car 1 on the internet and take a copy or screen shot.
4) Open another instance of Paint and paste in the picture
5) Use the square select tool to copy the 1st car and paste it below the 2nd car
6) Use the resize tool to stretch and shrink the car you have just pasted to the right size.
The resizing tool isn't that precise and I didn't bother with any measurements. In fact because I needed the whole car body to be a bit taller when I stretched it I ended up with oval wheels. Also you can see where I cut and paste bits of the car to change the length of the door as the reflection line is broken up into chunks. If you look closely you realise just what a crap job I did and if you look at what I acheived with the interior of the real car you will see I am not much better away from the computer.
It was really just a bit of fun rather than trying anything sensible. I am sure others on here can give you far better advice on doing proper diagrams.