Plan well, and include the writer in on the plan.
Prepare an editorial schedule with deadlines and a clear approval process. Otherwise, the writer is left uncertain of expectations, and the project's focus will be lost.
It is important to know why you hired the writer, and to assure him of your confidence. Provide him tools (lexicons for industry-unique terms, access to subject matter experts, etc.).
With good plans and a good process, it is then just a matter of getting things done.