Ask HN: Freelancers/Agencies, how do you write software specifications

I find it difficult to do any project that goes for more than a couple of days unless I have a locked down overall scope and preferably a detailed specifications document specially for web projects where we need to define things like UI, wireframes etc.

There are silo tools like balsamiq etc. that lets you do a wireframe. But then wireframes are just one part of the overall specs. What about things like Use cases, User Flow etc.

I am asking because if you do a big project that takes at least a month or so, it is dangerous to do them without clear specs. Clients mostly mean well but a loosely defined project can turn into a nightmare pretty quickly once the client gets into the mode of "I like this page from this site but the menu from this other site...".

Is there a tool that lets you do specs that actually work and keep you on track ? I know that there always will be scope creep but in my recent experience, a well defined specification makes your life much easier to keep both the client and project on track.