How to add Header names in xls files from sidekiq?

File.open("#{Rails.root}/tmp/report/sample.xls", 'w')
 { |f| f.write
 ( @policies) 
 } 

Above code is working fine its creating xls file in tmp folder, but i want header names in that xls file, how to pass header names in that code? this code written in sidekiq for background process. please any one suggest me