We assembled our quadcopter from a kit, which included an electronics package that basically just needed to be mounted and plugged in. Our primary reference was this website, which is easy to follow and full of comments that address many of the problems you’ll encounter (really, these forums and others you’ll find on the web are invaluable):
It’s no longer being maintained, though. The new site, while cleaner and easier to navigate, doesn’t seem to have the wealth of information yet.
For now, I would recommend using both in tandem.
UPDATE: as we posted earlier, 3D robotics has released this assembly manual. It would have saved us literally days of deciphering inferior instructions. Check it out.
Surprisingly, one of the biggest hurdles was understanding the language of our directions well enough to follow them. Before we even ordered the kit, I spent hours searching for the meanings of various acronyms, so to spare you the trouble, and to keep them in one place for myself, I’m listing them here. It’s strange, a lot of these have entered my day-to-day speech, but just a month or two ago they were completely opaque. That said, this might seem silly, but if I had been given the same list I could have been spared hours of google searches. I’ll probably keep adding to it as we learn new things: