1. Design a sedan.
2. Cut the sedan design: remove the 'B' pillar, remove the rear door, shorten the wheelbase some, add rear side glass...
From the French language, past tense of the verb 'couper', "to cut". More on the coupé body style on Wikipedia (makes for an interesting read).
3. Adjust the design a bit as necessary, don't worry too much about rear passengers.
And voilà, you have a coupé.
Drawings by Yours Truly ( www.pgcomics.com).