The old adage "a picture tells a thousand words" is rarely as important as it is here. Please, please, please, for your sake and for your users: USE LOTS OF SCREENSHOTS.
Having been in a similar situation, where most of my users were elderly women, I cannot convey how useful screenshots are. The question becomes, how to present them. I found that a task-based approach works best.
Table Of Contents
- Customer Information
- I want to update a customer's address
- I want to create an invoice for a customer
- I want to create a bill for a customer
Note that each heading in the TOC is obviously a link (which any format, be it word/html/chm/whatever supports). Then for each individual action (eg. 1.1) start with the first screen the app shows when it opens up.
- Start at the Home screen
- Click "Customers"
- [screenshot with big red circle around "Customers" button]
- You arrive at the Customers window
- [screenshot of the customers window]
- (note, use a test build with only fake customers)
- Click "Search"
- [screenshot with big red circle around "Search" button]
- Enter the customer's last name and double-click their name
- [screenshot of the search window with the customer's lastname in the search box]
- You arrive at the customer details screen
- [screenshot of the customer details]
- Click "Edit"
Yes, this first few steps are redundant, and the whole thing is lot of laborious screen-shooting. Users will get used to just flipping past the first page or two but anyone and everyone, on their first day on the job, or after a long vacation, or mat leave, or sick leave, or whatever... can be told to "Go do [this]" then click "I want to do [this]" in the docs and have detailed instructions showing exactly what to do.
You can have the first step be "Open the customer's file" which is linked to the "I want to see a customer's information" section. Be wary, I switched to that, and had a large portion of my users that found this extremely complicated. The ones who didn't find it complicated, didn't care enough to appreciate the 'shortcut'. The tip is: Know your audience and don't assume anything about them.
There's an old article from Joel Spolsky (that, for the life of me, I cant find) where he talks about usability and the wheelchair assistance bars in bathrooms. The crux of the argument was that some usability features aren't necessary for everyone... but when available, everyone uses and appreciates them. I firmly believe that incredibly obvious documentation for internal apps fits in this category. Especially considering that they can't be searched for help online.