The Douglas County Kansas Voting and Elections Department has lots of great information about voting in Douglas County. One particularly useful document is the list of which offices are on the ballot in which years.
They also have all of the important dates for this election year.
July 17, 2018: Last day to register for the Primary Election
July 18, 2018: Advance Voting by Mail begins for the Primary Election
July 31, 2018: Last day to request an advance ballot
August 6, 2018: Advance Voting closes at Noon
August 7, 2018: PRIMARY ELECTION