Remember all the processes learned in English Class and put them to use. Get a reference book and put it to good use. Keep in mind that documentation written may be translated to other languages or read by persons whose native tongue is not English. Clarity is of the utmost importance. It is also a good time to review the documentation thoroughly when one is reading it.
Good documentation takes a lot of time to write.
When one is intimately involved with the subject matter, they are more apt to pay attention to detail. It is not only important to list how to do something, but also why it works and what the command does. Making an end user more informed is the point of the document and thus no issue should be left untouched. Start the document by describing what the program does and document the command line arguments used by the program. A quick start guide is another good starting point for a manual.