I'm trying to document a piece of software I wrote in detail. An SAS is too high level and doesn't cover the API. I found several examples of an SDS online, but I don't see a trend in their format.
I'm wondering if there are some general guidelines to writing a Software Design Specification or what the best approach is?
You could do a lot worse than starting with the DOD-STD-2167A Software Design Document Data Item Description.
The US Department of Defense has a lot of experience with software development, a lot more than any one company. They also have a vested interest in actually getting software that did the job, and that could be maintained down the road. They went to a lot of trouble to figure out what information needed to be in the various documents that support a software project, using experience from a large number of projects, and more than a few very expensive failures.
The Software Development Plan DID (available here) includes a number of items that must be addressed. If you look closely, you'll notice that those sections are written in the blood of program managers who failed to address those items.
