DanThornton - 1:01 pm on May 8, 2012 (gmt 0)
It depends on what you want your writers to achieve.
I've spent 10 years writing for a living either full time, or freelancing alongside my SEO and Social Media work. I also work as a tutor to Journalism students on a top post-graduate course in the UK, so I've got fair bit of experience in what makes a good writer.
If you want the bare minimum, then some common sense guides to basic grammar and content briefs will do the job (Spell-checkers will cover typos).
But traditionally, spelling and grammar weren't part of the writer's job - they were part of the Sub-Editor role. One main reason is that it's a lot harder to spot mistakes in your own copy compared to checking something submitted by someone else.
And I'd completely disagree that you can't train a writer - I've been part of training people with no writing experience to become award-winning journalists, so it's very much possible. The bit you can't train is the passion and desire to write, and any good writer needs to be constantly curious. If they've got those two qualities, they could be writing for a national newspaper etc in 4-6 weeks of hard training.