Download an Absentee Ballot Request Form at the link below. It must be printed out, then filled out and signed and returned to your County Auditor. If you live in Story County the address is 900 6th St, Nevada, IA 50201. 

https://sos.iowa.gov/elections/pdf/absenteeballotapp.pdf

Iowa does not have permanent absentee voting. Voters must complete a request for an absentee ballot for each election for which they would like an absentee ballot.. Request forms will not be accepted more than 120 days prior to the election. Absentee voting can begin no more than 29 days prior to the election.
The deadline to request a mailed ballot is 11 days prior to the election. (10 days for a General election) NOTE: The request must be received in the Auditor's Office by 5 p.m. on the final day.