I'm going to assume you want to commission something fictional.
Using known prototypes to help illustrate what you want is generally the easiest way, though it might vary depending on just what you want to commission.
Let's say you're trying to describe the cab of a fictional steam locomotive you designed. Find a prototype with a cab shape that matches your design as closely as possible and describe whatever detail differences there might be. Side window shape and size (if applicable), forward lookout shape and size, roof shape, rear roof overhang, lettering, roof hatch, etc. All compared to prototypes. Don't be afraid to go too into detail.