First you should create a proposal template on your computer. Use any good word processor and include a company logo (if you have one) and full contact info. This template will be used for all proposals now and in the future.
Personally, I include all the areas that will be painted and all products that will be used. Here's a sample from an interior;
Painting the interior of your home to include the following;
Main Level – Entry (soffit), formal dining (ceiling and walls), kitchen/breakfast nook (walls and ceilings) and laundry room (walls and ceiling).
Basement – 2 hallways (walls and ceilings), walk-in closet (all), wine cellar (walls and ceiling), bathroom (walls), 2 bedrooms with closets (walls only in bedrooms and closets all), vestibule (walls and ceiling), recroom (walls and ceiling) and kitchen (walls and ceilings).
Wood Trim – Includes all removed base and door casing on the main level and all base and door casing in the basement. All new trim will be sanded smooth, stained with a matching wiping wood stain and finished with Coronado Aquaplastic acrylic urethane with a semi-gloss sheen. Approx- 370 ln/ft of base and 560 ln/ft of casing. All nail holes will be filled with color putty.
All walls and ceilings patched/retextured will be primed with an acrylic drywall primer.
All areas to be painted will have 2 coats of finish paint applied by brush and roller.
The walls will be painted with an eggshell sheen and the ceilings with a flat sheen.
Includes a separate color for the ceilings and a separate color for the walls, two colors.
I also include any options with a place to sign and date if accepting the option(s). The main signature for the entire job is a separate page with disclaimers; not responsible for acts of god etc; there are no verbal agreements regarding this contract, all verbal agreements, being merged herein; and that this constitutes the entire job anything extra is an additional cost to the customer. It's a long detailed legal page that limits the job and my responsibilities (liability).
Details are everything. If you leave anything out or are vague about the scope of work then you will have no recourse when the customer says "I thought this was a part of the job!"
Hope this helps.