Looks straightforward enough:
http://www.opm.gov/retire/pre/fers/disability.asp
You have to apply for both (and, presumably get SSDI or you won't get FERS); it reminds me some of a lot of State Income Taxes where they want you to copy your Federal information and then they just go from there. If you do not apply for SSDI, the FERS application gets cancelled too.