A user manual is the first line of defense against customer requests and problems. Customers might never read the manual when they first buy a product. However, when something goes wrong, it's the first place they go for help. This is the reason a user manual must be used for customer support.
Creating a user manual does require a lot of upfront time but once it's done, other than an occasional update, it's done. All areas should be easy to understand (readable) and clear as to the meanings and use of words. Customers should never question what is stated in a user manual. If you are not sure about the quality of your user manual, give it to someone who has never used the product and have them review the manual. They can easily point out the areas that need improvement.
A user manual needs to contain appropriate sections. Tangible products have a user manual; software products have a help section. Software can have a user manual but it's quite common for it to be built into the software. The primary difference between the two is software usually offers context sensitive help. Context sensitive help is help that is specific for the section of the software the customer is using. For example, the help information on the accounting software billing screen is specifically about the billing screen.
By writing a user manual with the proper sections, you are helping the customer solve their own problems. When you know what these sections are, writing it will be a breeze.