Here I explain how to make a convertible
As example this Alfa 147 GTA to make a convertible:

And this convertible to pick the roof off:

Select the entire roof, move it to the picture of the alf and set it on the right place:

Now you have to make sure that the roof (the part you can see) and the car are in the same color, I colored now the entire color, because I didn't like the white:

Now you have to put it on another backdrop, and put it at the right place:

Now the backdrop has to come through the frontwindow(see backdrop change topic if you don't know how to do that) and you're ready:


BTW: This was the backdrop I used: