Alot of times from what i have seen ( and its a big stress reducer) Alot of people measure top to bottom and left to right and make a square print... Then apply and trim excess off... (example a)
But if you are want to be specific to that type of side... For example I had an old Universal Mr.Do cab that had particular lines and curves I wanted to copy... I took a roll of old poster paper and covered the side and traced out the design... I then took the sheet cut it into strips, marked them and scanned them using an old bed scanner... I then put it back together thru Photoshop and then vectored the design via Illustrator...
(example B)
Note, its not perfect but with what i had, it suited my needs...
Hope this is what ypu are trying to ask...Good luck