These are guides that cover many different aspects of legal writing. If you are looking for a single helpful source, one of these guides may fit your need.
Legal writing requires you to express arguments using legal reasoning. These sources explain how to do that effectively. They tend to focus on brief writing.
The writing of contracts and legislation differs substantially from the writing of legal arguments. These sources can provide insight into these particular types of legal writing.
Academic legal writing has a different purpose and format than other types of legal writing. These sources may give you some guidance on writing academic pieces.