Ask HN: Advice for creating design documentation.

Context: I'm a young developer that's been asked to write up a design document for my next feature.

My Experience: I've only been exposed to design documentation in school for assignments as well as the samples at work.

Do you have any habits that you've formed when creating documentation? What do you know now that you wish you knew earlier? Anything will help, I just want to add some polish to my work.