Technical writers use a variety of sources to develop their documentation:
The first source of information is the product itself. To write an instruction guide or user manuals, the writer assembles and uses the product. This allows the writer an opportunity to detect any problems or confusion the audience might have. It also lets the writer implement solutions to these problems by testing them first hand.
Reference books are a good source of information for technical writers. Usually, the reference contains general information that guides a writer through transition material needed to explain broad subjects or concepts. The writer must be very cautious when using unauthorized Web references. Ask the SME if a Web site is a credible source for information.
The client might provide the writer with old technical documents to help illustrate the overall style and philosophy desired. The client usually arranges access to an SME for additional help. Technical writers interview experts and users, and gather as much information as possible to clarify facts.
Technical writers often evaluate competitors' products. Most software programs have several competitors. The writer can use the competitor's text for a resource. Of course, plagiarism is never okay. Never copy a competitor's material.
Technical writers working on scientific material will find professional journals helpful. Journal articles are written by scientists for their peers. They are rife with technical jargon and are written at a university graduate level. Carefully extract information and translate the technical jargon into plain language. Adapt journal articles by following the Federal Plain Language Guidelines, available at http://www.plainlanguage.gov/especiallyfor/writers.cfm.