You can go to the embassy and they will give you the form and mail it for you. Voting deadlines vary according to your state of residence. Make sure that you have you address in the US and your driver's license number. (I didn't and had to make two trips.)
I think that there is going to be an "election day" at the USA embassy/consulate on Wednesday October 8th. But you have to send in your stuff AND put your name on a list at the embassy as someone who wants to vote on the 8th.