I usually start by searching on the web for basic concepts and gathering the information. Generally I prefer to look for people who are experts or related to the subject area and look for a slot of their time to catch their insight. I also look for documents which might be similar or related to the one I have to produce.

For step two I have some basic templates for the types of documents I have to produce, so I just brainstorm and fill the template up.

Once I feel I have enough information and have a decent mind map of the subject area in my brain, I start to organize the information and write the draft of the document.

I always begin by visiting with my client.  We discuss the project's needs, parameters, goals, missions, etc.  Then, I make a mental outline (just a quick list of what components the project will included).  

After this list, I start filling in the blanks, drafting the text.  Of course, to draft, I do research (usually online).  The research not only lets me see what's new in the field but also research provides checks and balances for me.  That is, I want to make sure that my thoughts on a project are correct before I proceed.

After the draft, I read and re-read the text, looking for holes.  When I find areas that haven't been fully developed, I add new information.

The final phase is to get feedback from my client.  They review a draft to make sure what I have written fits their needs.  This leads to revision and implementation.