What is your process, for analyzing and writing specifications of a project before coding?

I was recently asked to analyse a project and write a little brief / specification, but the thing is that I don't really know how to do it.

At this point, I have extracted the main features, and destructed them in tiny tasks, thought about the architecture of the project, and which technology might suit best for the needs.

Do i have some other things to analyse ? How do you write this in a meaningful way ?

Thank you in advance Have a nice day!