Here's an email reply from my township clerk a couple of years ago. This was in Michigan. The rules may vary from state to state I think. Not sure. "Good Morning: I need to receive an application for an AV ballot. You can print the application off, sign and scan it back to me. Here is the military & overseas app: https://www.fvap.gov/uploads/FVAP/Forms/fpca2013.pdf. Or you can mail me an AV application/letter -I need your signature to verify. You need to let me know how you want to receive your ballot on the application (email, fax or mail) . Then you will have to mail the voted ballot back to me. Thanks Dawn LaLonde Alcona Twp Clerk" There is also information on AV at this U.S. Embassy Manila website: Home | Manila, Philippines - Embassy of the United States
Each state has their own election board website, go there and you should be able to find out how to apply for your absentee ballot.
fvap.gov is the place to go, it's run by the Pentagon. While living in Guam for 8 years we lost the right to vote in Stateside elections. The Fvap agent called our previous County election official and got them to put us on the rolls for absentee voting.