I visit a lot of customers and I hear many of the same questions. One of the most common at the beginning of a project is how do I document and design the transformations.
I have created a template (that you can download) that I use and provide when I am on a project to the architects, developers and business analysts. I find that the business analysts are typically in the best position to do this part of the analysis and they are most often very comfortable with an Excel spreadsheet.
Each map will have an associated Excel spreadsheet. Within the spreadsheet are 4 tabs. The first contains history information such as the author, date, version and description. The second and third tab are the mappings for a request and response message scenario. If this map represents a one way message then delete one of the tabs. On these mapping tabs I have included the source schema information in column B, destination schema information in column J (there is other data on the form pertaining to the schemas) and the transform information in column G. Column G maps to the fourth tab which represents the transformation functions.
In the template, I have included a sample of a public schema mapped to a canonical schema as well as a sample response message mapping. On each of the tabs I have included examples of the types of mapping that you will encounter including:
Mapping from source to destination
Creating a destination value with no mapping from the source
Mapping source to destination through the use of a functoid
Mapping a source element to two destination elements