Just call them up and tell them you want to recharacterize. Make sure you use that word. It's less important in the Roth -> traditional direction, but at least in the opposite direction it would be easy for them to misinterpret and make a conversion if you used words like transfer. I'm 90% sure I've never filled out the form SweatingInAZ linked to, so I'm pretty sure it can all be done over the phone.
Alternatively, it looks like if you log in and then search "recharacterize" in the search box in the upper right the first search result is an online form you can fill out. I can't remember if I've ever done this, so I'm not sure what happens once you complete it (if it just works, you still have to call in, or if it gives you a form to mail in). Report back if you use it.
You can recharacterize any time until your tax filing deadline. If you might not want to recharacterize the whole thing I'd wait until you get final income numbers (wages, interest, dividends).
Make sure you attach a statement to your taxes explaining what you've done as directed in the instructions for form 8606.