Ask HN: Examples of good technical design docs

As part of an interview process, I have recently received a take home assignment of writing a technical design doc for a simple web app. Turns out I badly suck at it.

I have found resources describing the templates, the "hows" and "whys" of TDD, but I struggle to find some actual, real-life examples.

Can you recommend any good technical design docs? Or any other resource that would make you better at writing such documents?