This page appears to describe the standarsized process for registering to vote as an overseas absentee voter: www.fvap.gov/citizen-voter/

It states that whether or not you just want to vote for federal offices, the important criteria is to establish your “voting residency”, and then the elections office of that county would send you a ballot for what you’re eligible to vote for.

I can’t imagine this will be simply to navigate, but I applaud you for seeking to exercise your right to vote. Good luck!