There is a process I go through in making my typical car portrait.  Keep in mind that most classic car owners do not drive their car on a daily basis, nor are they willing to transport or drive it to vintage roadside locations.  Therefore, my typical car portrait is not what it appears to be.  Most are produced from multiple photos using Adobe Photoshop CC.  The process is usually as follows.

FIRST, travel to vintage locations and amass a collection of background photos suitable for the task.  This requires photos shot from many different angles and ideally with different angles of light and lighting conditions.

SECOND, take many photos of the automobile from many different angles and ideally with different angles of light.

THIRD, cut the automobile out of the original photo using the “pen” tool.  Next, cut the glass out, making sure that none of the original photo background can be seen through the cutout of the automobile.  Replace the glass using a suitable color from the pallet and add the appropriate amount of noise to match any remaining glass in the photo.  Adjust the transparency so that the new background is visible through it.  Use the “blur” tools as required to make it seem natural.  Make sure that there are no outlines at the edges of the cutout. Many times, they can be shaded to match, if cutting won’t take them out.  Address any issues with reflections of original background environment from the chrome, paint, and remaining glass.  Make sure that the vehicle and the newly replaced glass are each in separate layers.

FOURTH, cut the shadow of the vehicle from the original photo using the “pen” tool.  Save the area to a new layer and paint it solid black.  This will be used as a selection area to create a new shadow using a brush or the gradient tool or combination of two in order to reach the desired effect with the new background.

FIFTH, decide which background will be used for the portrait. Remember to make sure the vehicle is placed so that it lines up properly with the vanishing point of the background’s horizon.  Scale, rotate, and place each in order to create correct perspective.  If not, you may have to find another background.  It usually works best if you know which background you want to use before taking the photos of the automobile.  That way,you can study the background and try to place the automobile, so it all works out.  Make sure the shadows cast in the background can be matched to the photo of the vehicle or at least close enough to adjust.  

SIXTH, fine tune the various layers, shadows, automobile, and background until everything looks perfect.  

Keep in mind that these are only the basic steps involved in the creation.  There are many others that can be unique to each creation before the desired results are obtained.